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Crop factor affects maximum F/stop?
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May 1, 2021 23:26:45   #
flyboy61 Loc: The Great American Desert
 
Educate me, please! I was just reading some information on the Tamron 90mm f/2.8 Macro lens that I am thinking of buying, in which someone stated that on a DX camera, the effective field of view with a FX lens, would be ~ a 135mm equivalent, but the crop factor would also affect the f/stop, raising it to f/4.2.

Not saying this isn't true, but it is the first time I have heard that, and it doesn't seem correct to me. Despite the narrower field of view, the distance from the front element of the lens to the sensor doesn't change, and the light has no further to travel than before, so the f/stop should remain the same. (?)

Next, and I admit this is something I have never given much thought to, but lenses with internal focus do not change their physical length, therefore light has no further to travel from the lens' front element to the sensor, which I understand is the reason for non-internal focus macro F/2.8 lenses to have an actual rating of ~ f/4 at 1:1 distances, when their lens barrels are extended "waaay out yonder".

That's the reason my 70-300mm non-IF zoom is placarded at F4.5-5.6, and my 70-200 I F zoom is a fixed F/4 throughout their zoom ranges. So, providing my understanding is correct, there should need to be no "adjustment" of the f/stop at close focus distances for Internal Focus lenses. Or?

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May 1, 2021 23:59:25   #
TriX Loc: Raleigh, NC
 
You are correct. An f2.8 is an f2.8 regardless of what format camera it’s used on. It’s the physical ratio of the focal length to the entrance pupil diameter and neither change regardless of what sensor the image is projected onto.

This debate surfaces periodically on UHH, and proponents of the idea that the f stop changes based on the sensor size refuse to believe otherwise despite the actual definition, the fact that the lens yields the same exposure on both FF and crop bodies, or the fact that light meters don’t have separate scales for various sensor sizes.

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May 2, 2021 00:01:52   #
Wallen
 
flyboy61 wrote:
Educate me, please! I was just reading some information on the Tamron 90mm f/2.8 Macro lens that I am thinking of buying, in which someone stated that on a DX camera, the effective field of view with a FX lens, would be ~ a 135mm equivalent, but the crop factor would also affect the f/stop, raising it to f/4.2.

Not saying this isn't true, but it is the first time I have heard that, and it doesn't seem correct to me. Despite the narrower field of view, the distance from the front element of the lens to the sensor doesn't change, and the light has no further to travel than before, so the f/stop should remain the same. (?)

Next, and I admit this is something I have never given much thought to, but lenses with internal focus do not change their physical length, therefore light has no further to travel from the lens' front element to the sensor, which I understand is the reason for non-internal focus macro F/2.8 lenses to have an actual rating of ~ f/4 at 1:1 distances, when their lens barrels are extended "waaay out yonder".

That's the reason my 70-300mm non-IF zoom is placarded at F4.5-5.6, and my 70-200 I F zoom is a fixed F/4 throughout their zoom ranges. So, providing my understanding is correct, there should need to be no "adjustment" of the f/stop at close focus distances for Internal Focus lenses. Or?
Educate me, please! I was just reading some inform... (show quote)


Do you have one of those flashlights that can zoom? The one in which you can adjust the light to be bigger or smaller in diameter? That can explain you dilema.
The flashlight throws out the same amount of light. Make the diameter small and it becomes bright. Make it big and it become dull.
In a camera lens, if everything stays the same except for the sensor size, if you view that in their natural state, there would not be any change in brightness in the image. you only get a cropped photo.

But if you print the image from the smaller sensor, to equal the bigger sensor, then your print will appear as if it was taken in a darker situation in comparison. Just like the small light from the torch spreading out to a bigger diameter.
In photography., we see the change as a higher f stop number equivalent.

The smaller sensor size is getting the same amount of light, but it is being distributed to a higher density smaller pixel wells. Much like a pizza sliced to share with 50 people (apsc) and 25 people (full frame). Its the same pizza, but they will not get the same size of slice. The 50 people will get a smaller bread to bite.

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May 2, 2021 00:07:38   #
Rongnongno Loc: FL
 
Wallen wrote:
.../...

Uh?

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May 2, 2021 00:13:56   #
fjdarling Loc: Mesa, Arizona, USA
 
There may be other well-known photographers who agree with that idea, Tony Northrup is definitely among them. It's been a while since I viewed his YouTube video on the subject, but I believe his assertion was that the smaller sensor was capturing only the light in the center of the lens's projected "image", which suggests that some of the light is lost or wasted, resulting in an effective aperture smaller than expected. There should be a way to test that theory, but I can't think of one right now. If there is a way, it might require both a full-frame and a cropped-sensor cameras that will mount the same lens so a comparison of the settings can be made. I suggest you search YouTube for 'Tony Northrup crop factor aperture' if you'd like to hear his explanation.

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May 2, 2021 00:27:13   #
larryepage Loc: North Texas (Dallas area)
 
As Trix has stated, f/stop is simply the ratio of two geometric parameters of the lens. It does not care what the image formed by the lens is projected onto or how much of it is used.

"Focus Breathing," an imagined distortion of the term "magnification," is a a mostly real phenomenon, but it does not result from the change in the presumed focal length of a lens. Focal length is also a physical characteristic of a lens, and it is constant for any non-zoom lens. It is common in photography to estimate the magnification of a lens by dividing the focal length of the lens being used by the focal length of a "normal" lens. This is an estimate only, and it is only good at a particular distance, usually what we call "infinity." Of course, an observant person immediately realizes that there are two big problems with this...the first is that different people have different ideas of what constitutes a "normal" lens. I know people who claim anything from 40mm to 55mm (or even 58mm) as normal for a full frame camera. The standard is the length of the diagonal of the image. And of course, most of what we shoot, even distant mountains, is not an infinite distance away.

But...back to magnification...the real calculation for magnification (m) is given as
m = i/o (or image distance divided by object distance)
The complication is that in order to focus closer, the lens (or some part of it) has to move away from the sensor. For lenses that focus really close, whatever moves has to move quite a long way. This upsets the arithmetic a little bit and causes unexpected changes in the magnification. The changes are sometimes different in internal focus and non-IF lenses. But it isn't unexpected, and it's nothing sinister.

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May 2, 2021 00:31:44   #
Wallen
 
Rongnongno wrote:
Uh?


LOL

Imagine a room with 50 people, we call that full frame.
Imagine another room only half the size and contain the same amount of people. That is our APS-C

The other room being only half the size, if we compare size for size equality, our full frame room only has 25 persons in it while the APS-C has 50.

If i deliver one pizza to an equal room size basis, then 1 pizza get to be shared with 25 people in the fullroom and 50 people in the APS-C.

That is what happen to light going on a lens. It's the same f-stop. Same pizza. Same amount of light, but it is being shared by a lot more pixel. So the pixels receive a smaller amount of light, same as the small room where each person gets a smaller slice of pizza.

That amount to an exposure equivalent that is more noisy compared to the full frame. Specially in lower light conditions.

How do we get the same quality? More light. So any full frame setting needs to be set 1 stop lower for the APS-C to have the same image quality.
We'll we cant have more light because we take a picture of the same event with 2 different cameras.
So we end up with the APS-C having a "practical" equivalent of a higher f-stop number.

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May 2, 2021 00:32:39   #
TriX Loc: Raleigh, NC
 
Wallen wrote:
Do you have one of those flashlights that can zoom? The one in which you can adjust the light to be bigger or smaller in diameter? That can explain you dilema.
The flashlight throws out the same amount of light. Make the diameter small and it becomes bright. Make it big and it become dull.
In a camera lens, if everything stays the same except for the sensor size, if you view that in their natural state, there would not be any change in brightness in the image. you only get a cropped photo.

But if you print the image from the smaller sensor, to equal the bigger sensor, then your print will appear as if it was taken in a darker situation in comparison. Just like the small light from the torch spreading out to a bigger diameter.
In photography., we see the change as a higher f stop number equivalent.

The smaller sensor size is getting the same amount of light, but it is being distributed to a higher density smaller pixel wells. Much like a pizza sliced to share with 50 people (apsc) and 25 people (full frame). Its the same pizza, but they will not get the same size of slice. The 50 people will get a smaller bread to bite.
Do you have one of those flashlights that can zoom... (show quote)


Nope. With the flash light analogy, you’re actually changing the luminance per area by concentrating the light, but that’s not what happens with a camera lens. In that case, unlike the flashlight, whose illuminated area gets smaller, the projected area for the lens remains the same, but only a percentage is used to illuminate the sensor - the luminance per area does not change.

You can easily prove this to yourself by using the same lens on a crop and a FF camera at the same exposure settings, and the resulting image will be the same brightness. Or you can use a lightmeter, meter the same scene, set the same settings and expose that same image on two different format cameras (film or digital) and look at the resulting brightness/density and they will be the same if the shutters are accurately calibrated.

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May 2, 2021 00:40:16   #
Wallen
 
TriX wrote:
Nope. With the flash light analogy, you’re actually changing the luminance per area by concentrating the light, but that’s not what happens with a camera lens. In that case, unlike the flashlight, whose illuminated area gets smaller, the projected area for the lens remains the same, but only a percentage is used to illuminate the sensor - the luminance per area does not change.

You can easily prove this to yourself by using the same lens on a crop and a FF camera at the same exposure settings, and the resulting image will be the same brightness. Or you can use a lightmeter, meter the same scene, set the same settings and expose that same image on two different format cameras (film or digital) and look at the resulting brightness/density and they will be the same if the shutters are accurately calibrated.
Nope. With the flash light analogy, you’re actuall... (show quote)


Yep, they will expose the same but the smaller sensor will have lower IQ.
See the pizza explanation.

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May 2, 2021 00:45:05   #
TriX Loc: Raleigh, NC
 
First flashlights and now pizzas? How about instead you actually use the definition (https://en.m.wikipedia.org/wiki/F-number) and consider the concept that the luminance per area does not change. Better yet, perform the experiment I suggested. Have you actually tried using the same lens on a two different format cameras with the same exposure settings and compared the images? I actually have (using the same lightmeter measurements and exposure settings), and I KNOW the answer even if I didn’t believe the definition. You should try it.

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May 2, 2021 00:52:09   #
TriX Loc: Raleigh, NC
 
Wallen wrote:
Yep, they will expose the same but the smaller sensor will have lower IQ.
See the pizza explanation.


So you agree they have the same f stop and the same exposure? Well, that’s a start.

IQ is a different question, but you cannot accurately make that blanket statement either. If the pixel density is such that they both expose the same number of pixels, and the sensor technology is the same, the smaller sensor may actually have better IQ since it’s using the center of the lens where there is less vignetting and lower distortion.

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May 2, 2021 01:12:08   #
wdross Loc: Castle Rock, Colorado
 
flyboy61 wrote:
Educate me, please! I was just reading some information on the Tamron 90mm f/2.8 Macro lens that I am thinking of buying, in which someone stated that on a DX camera, the effective field of view with a FX lens, would be ~ a 135mm equivalent, but the crop factor would also affect the f/stop, raising it to f/4.2.

Not saying this isn't true, but it is the first time I have heard that, and it doesn't seem correct to me. Despite the narrower field of view, the distance from the front element of the lens to the sensor doesn't change, and the light has no further to travel than before, so the f/stop should remain the same. (?)

Next, and I admit this is something I have never given much thought to, but lenses with internal focus do not change their physical length, therefore light has no further to travel from the lens' front element to the sensor, which I understand is the reason for non-internal focus macro F/2.8 lenses to have an actual rating of ~ f/4 at 1:1 distances, when their lens barrels are extended "waaay out yonder".

That's the reason my 70-300mm non-IF zoom is placarded at F4.5-5.6, and my 70-200 I F zoom is a fixed F/4 throughout their zoom ranges. So, providing my understanding is correct, there should need to be no "adjustment" of the f/stop at close focus distances for Internal Focus lenses. Or?
Educate me, please! I was just reading some inform... (show quote)


The f-stop calculated for an exposure is the f-stop one uses for exposure. It does not change for determining exposure with a change in format. But depth of field is a different story. The set f-stop for an exposure will not change for any format but the depth of field, for that same f-stop and related exposure, will change for each format.

And internal focus or not, as any lens that approaches 1:1 loses the same amount of light as any other lens. One must adjust the exposure only for the magnification as one gets close to 1:1 magnification if one is doing an external manual exposure determination. The internal camera meter should be able to determine your exposure properly. Also be aware that camera movement affects the exposure the closer one gets to 1:1. Macro shooting is actually harder to shoot handheld than super telephoto is at large magnifications.

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May 2, 2021 01:20:00   #
GoofyNewfie Loc: Kansas City
 
fjdarling wrote:
There may be other well-known photographers who agree with that idea, Tony Northrup is definitely among them. It's been a while since I viewed his YouTube video on the subject, but I believe his assertion was that the smaller sensor was capturing only the light in the center of the lens's projected "image", which suggests that some of the light is lost or wasted, resulting in an effective aperture smaller than expected. .. I suggest you search YouTube for 'Tony Northrup crop factor aperture' if you'd like to hear his explanation.
There may be other well-known photographers who ag... (show quote)

Tony does have have a lot of good information, but the high production quality in his video doesn't make it correct in this case.
A smaller sensor doesn't change the exposure.

Period.

I've stated this before and it's been said above:
A hand-held light meter DOES NOT have different exposure compensation scale for different size formats because...
....it's not necessary.
I've shot film formats from 35mm through 8 x 10 inches.
Used the same light meter for all those formats.
No compensation needed.

It’s been a while since I’ve seen it, but I don’t think he mentioned full -frame equivalence at all in the video

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May 2, 2021 01:47:24   #
wdross Loc: Castle Rock, Colorado
 
flyboy61 wrote:
Educate me, please! I was just reading some information on the Tamron 90mm f/2.8 Macro lens that I am thinking of buying, in which someone stated that on a DX camera, the effective field of view with a FX lens, would be ~ a 135mm equivalent, but the crop factor would also affect the f/stop, raising it to f/4.2.

Not saying this isn't true, but it is the first time I have heard that, and it doesn't seem correct to me. Despite the narrower field of view, the distance from the front element of the lens to the sensor doesn't change, and the light has no further to travel than before, so the f/stop should remain the same. (?)

Next, and I admit this is something I have never given much thought to, but lenses with internal focus do not change their physical length, therefore light has no further to travel from the lens' front element to the sensor, which I understand is the reason for non-internal focus macro F/2.8 lenses to have an actual rating of ~ f/4 at 1:1 distances, when their lens barrels are extended "waaay out yonder".

That's the reason my 70-300mm non-IF zoom is placarded at F4.5-5.6, and my 70-200 I F zoom is a fixed F/4 throughout their zoom ranges. So, providing my understanding is correct, there should need to be no "adjustment" of the f/stop at close focus distances for Internal Focus lenses. Or?
Educate me, please! I was just reading some inform... (show quote)


I have viewed Tony Northrup's video on the aperture for exposure changing with format and know for a fact it is not true. If I determine an exposure with my handheld light meter to be f4, 1/60 second, and ISO 100 (or ASA 100 film), I will set my Hasselblad 503CW, Canon A2e, Olympus OM-1, Olympus OM-4Ti, Olympus E-M1 mkII, or Olympus XZ-1, and manually set those four different format cameras, to f4, 1/60 second, and ISO (ASA) 100 and I will get the exact same exposure from each of the four formats. But my depth of field for each of the four formats will be totally different. I have seen some of Tony's videos and, being a test inspector in the space industry, his testing methods and comparison methods are not even close to the likes of DPReview or Image Resource in accuracy and truth. I have seen some people take Tony's videos for "fact" even when they can be easily dissected as not acturate. Some of Tony's videos are OK, but one must be aware that some of his conclusions from his "testing" methods leave a lot to be desired.

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May 2, 2021 03:14:54   #
Wallen
 
TriX wrote:
So you agree they have the same f stop and the same exposure? Well, that’s a start.

IQ is a different question, but you cannot accurately make that blanket statement either. If the pixel density is such that they both expose the same number of pixels, and the sensor technology is the same, the smaller sensor may actually have better IQ since it’s using the center of the lens where there is less vignetting and lower distortion.


We can make it a blanket statement because it's just the natural physics of things. But as always, there would be a few flyers up or down & extremes somewhere by technology or design or maybe just a unknown fluke/luck of the moment.

Going back to the blanket statement and referring to the pizza explanation, It can't be equally the same. If we had the same number of pixels, per size(same pixel density or pixel per inch), then the FX (fullframe) at its natural form will have twice the number. This would be like comparing a D850 to a D7200. They have almost the same pixel density but the FX having much more area will have much more pixels and better IQ.

If we have the same number of pixel but not equal area like a D610 to a D7200 and shoot at the same distance & setting, then we can expect more detail and apparent zoom from the D7200. If that was a perfect glass, and perfect lighting condition & exposure, it would certainly be a better IQ.

But if the lens was so-so, just ok for FX, it will be a bad lens in the DX because whatever flaw it has will be multiplied 2X. A good example is using a Sigma 70-300 mm F4-5.6 DG Macro Lens on an APS-C camera.
It works well in the FX but it is soft when used on a DX body.
More gremlins appear when the lighting is not good. Even if we were able to expose well, it will still be a less IQ image in the DX because of the smaller pixel size, getting a smaller share of the pizza.

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