Ugly Hedgehog - Photography Forum
Home Active Topics Newest Pictures Search Login Register
Main Photography Discussion
mathematical challenge :)
Page 1 of 4 next> last>>
May 4, 2019 11:46:25   #
blackest Loc: Ireland
 
Ok here is a bit of madness
I have a Pentax k1 which shoots a full frame image of 7360 x 4912 pixels
A nikon D7200 has a crop sensor of 6000 by 4000 pixels.

Lets say the nikon uses a 200mm lens
with what focal length would i have to use on the K1 so that if I crop to 6000 by 4000 it has the same field of view as the photo taken by the D7200?

I think the crop factor is 1.23 which would be around 246mm but i'm probably wrong :)

Reply
May 4, 2019 11:52:46   #
lsupremo Loc: Palm Desert, CA
 
Does anyone care?

Reply
May 4, 2019 12:07:06   #
Rich1939 Loc: Pike County Penna.
 
lsupremo wrote:
Does anyone care?


Obviously

Reply
 
 
May 4, 2019 12:11:53   #
BebuLamar
 
250mm.
I hope the OP posted the question so he does care.

Reply
May 4, 2019 12:12:27   #
Rich1939 Loc: Pike County Penna.
 
blackest wrote:
Ok here is a bit of madness
I have a Pentax k1 which shoots a full frame image of 7360 x 4912 pixels
A nikon D7200 has a crop sensor of 6000 by 4000 pixels.

Lets say the nikon uses a 200mm lens
with what focal length would i have to use on the K1 so that if I crop to 6000 by 4000 it has the same field of view as the photo taken by the D7200?

I think the crop factor is 1.23 which would be around 246mm but i'm probably wrong :)


Would you be wanting to crop by the lens you choose or in post?
and, wouldn't the crop factor be 1.5 (no matter the pixel count)

Reply
May 4, 2019 12:13:13   #
pquiggle Loc: Monterey Bay California
 
I got 245. To obtain the same field of view using the entire sensor area you would need a 300mm on the Pentax. Then cropping the image from the Pentax by 1.23 would give an effective focal length of 300/1.23 which is 245mm.

Reply
May 4, 2019 12:18:31   #
Longshadow Loc: Audubon, PA, United States
 
Using a 200 on a 1.23 crop (D7200-smaller sensor) would be like a 246 on a full frame (K1-large sensor).
Going backwards, to get the effect of the same image from the crop sensor, wouldn't you need a 162 on the full frame? (200/1.23)

Full=Crop*1.23 and Crop=Full/1.23

Reply
 
 
May 4, 2019 12:19:49   #
BebuLamar
 
pquiggle wrote:
I got 245. To obtain the same field of view using the entire sensor area you would need a 300mm on the Pentax. Then cropping the image from the Pentax by 1.23 would give an effective focal length of 300/1.23 which is 245mm.


It's not exact because there is something not quite right in the specs of both camera. So yes 245-250 about correct I would say.
On the D7200 they said the sensor is 15.6x23.5 with 4000x6000 pixels. So is it 255 or 256 pixels per mm?
Same thing on the K1 it's either 204 or 205 pixels per mm.

Reply
May 4, 2019 12:26:50   #
Rich1939 Loc: Pike County Penna.
 
blackest wrote:
Ok here is a bit of madness
I have a Pentax k1 which shoots a full frame image of 7360 x 4912 pixels
A nikon D7200 has a crop sensor of 6000 by 4000 pixels.

Lets say the nikon uses a 200mm lens
with what focal length would i have to use on the K1 so that if I crop to 6000 by 4000 it has the same field of view as the photo taken by the D7200?

I think the crop factor is 1.23 which would be around 246mm but i'm probably wrong :)


the angle of view for a 200mm on a DX lens is 6.7 deg. The closest full frame is 300mm at 6.9 deg. The crop factor is not 1.23 it is 1.5 just like any full frame vs Nikon dx. What ever lens you mount will not crop your image it will fill the frame. Unless your camera has a dx setting in which case the question needs to be rephrased

Reply
May 4, 2019 12:35:43   #
Bill_de Loc: US
 
Rich1939 wrote:
the angle of view for a 200mm on a DX lens is 6.7 deg. The closest full frame is 300mm at 6.9 deg. The crop factor is not 1.23 it is 1.5 just like any full frame vs Nikon dx. What ever lens you mount will not crop your image it will fill the frame. Unless your camera has a dx setting in which case the question needs to be rephrased




Pixel count does not determine field of view. If it did a full frame camera and a dx camera with the same pixel count would have the same field of view with the same lens. Think D5 and D500. The physical sensor size is what makes the difference.

--

Reply
May 4, 2019 12:36:43   #
dcampbell52 Loc: Clearwater Fl
 
Rich1939 wrote:
Would you be wanting to crop by the lens you choose or in post?
and, wouldn't the crop factor be 1.5 (no matter the pixel count)


You're mostly correct. I was going to reply to someone that was asking about crop or DX lenses or Full frame lenses.
In answer to that and kind of in answer to this, a crop lens just means that the cone of light reaching the sensor is just large enough to cover the DX sensor. On a Nikon FX or DX Camera, you can use either full frame lenses.. I have an 80-400mm Nikkor Full Frame and a Nikkor 50mm f/1.2 full frame. I use these interchangeably on my Nikon D7100 (crop sensor DX) and my Nikon D610 full frame. I also have a Tamron 10-24mm DX, Nikkor 18-70mm DX lens and a Nkkor 70-300mm lens. These can also be used on either the Full Frame Nikon D610 or the DX Nikon D7100. However, on the Full Frame D610, it will either give a rounded edge image at the limits of the DX to FX on the sensor or I can switch the D610 to DX mode. The DX camera just takes the center (DX) portion of the FX image and ignores the rest. So all of my lenses can be used on either DX or FX... However, the DX lenses on the FX camera will still only display the DX portion of the image and gives the 1.5 (roughly) magnification. The FX lenses on the DX camera will also work but the DX sensor will ignore any part of the image outside of the sensor and will also do the 1.5 (roughly) magnification.

Reply
 
 
May 4, 2019 13:00:37   #
John_F Loc: Minneapolis, MN
 
Pixel is just a name for clusters of charge centers to record binary digits. Pixel is not a measure of physical size. The standard lens equation and magnification formula enable angle of view. For this, you need the sensor dimension actually used to record the binary image data.

Defining s and s’ as the distances from the front lens principal planes (mathematical planes) and sensor dimension a (in subject space) and a’ actual size, we obtain

a/a’ = s/s’ then using the lens equation 1/s + 1/s’ = 1/f eliminate 1/s’ yielding an equation to evaluate a.

From a and s computer the angle of view from the tangent rule.

Reply
May 4, 2019 13:06:18   #
blackest Loc: Ireland
 
Rich1939 wrote:
the angle of view for a 200mm on a DX lens is 6.7 deg. The closest full frame is 300mm at 6.9 deg. The crop factor is not 1.23 it is 1.5 just like any full frame vs Nikon dx. What ever lens you mount will not crop your image it will fill the frame. Unless your camera has a dx setting in which case the question needs to be rephrased


Let me try and clarify a bit
a k1 is 36Mpix full frame and a d7200 is 24Mpix aps-c.
If I put the K1 in Aps-c mode then it will have the same angle of view as the D7200 with a 200mm lens but it will be a little less than 16 Mpix in size. So the fella with the D7200 says he has more megapixels in his image since he has 24 Mpix.

Ok so now I crop my photo to 6000x4000 same as the D7200 but the field of view is wider. So I need to pick another focal length that gives me the same field of view as the d7200 when I crop to 6000x4000.

I would have the same Mpix count in my image plus a bit of leeway in where exactly I take the crop. If I use 300mm I wouldn't have this. So what focal length do i need.

Reply
May 4, 2019 13:16:27   #
BebuLamar
 
blackest wrote:
Let me try and clarify a bit
a k1 is 36Mpix full frame and a d7200 is 24Mpix aps-c.
If I put the K1 in Aps-c mode then it will have the same angle of view as the D7200 with a 200mm lens but it will be a little less than 16 Mpix in size. So the fella with the D7200 says he has more megapixels in his image since he has 24 Mpix.

Ok so now I crop my photo to 6000x4000 same as the D7200 but the field of view is wider. So I need to pick another focal length that gives me the same field of view as the d7200 when I crop to 6000x4000.

I would have the same Mpix count in my image plus a bit of leeway in where exactly I take the crop. If I use 300mm I wouldn't have this. So what focal length do i need.
Let me try and clarify a bit br a k1 is 36Mpix fu... (show quote)


And I say the result you got of 246mm in your initial post was correct.

Reply
May 4, 2019 13:47:05   #
Rich1939 Loc: Pike County Penna.
 
blackest wrote:
Let me try and clarify a bit
a k1 is 36Mpix full frame and a d7200 is 24Mpix aps-c.
If I put the K1 in Aps-c mode then it will have the same angle of view as the D7200 with a 200mm lens but it will be a little less than 16 Mpix in size. So the fella with the D7200 says he has more megapixels in his image since he has 24 Mpix.

Ok so now I crop my photo to 6000x4000 same as the D7200 but the field of view is wider. So I need to pick another focal length that gives me the same field of view as the d7200 when I crop to 6000x4000.

I would have the same Mpix count in my image plus a bit of leeway in where exactly I take the crop. If I use 300mm I wouldn't have this. So what focal length do i need.
Let me try and clarify a bit br a k1 is 36Mpix fu... (show quote)


A lens that will get you close is probably around 240-250 mm. Since using crop mode will give a image size less than you want. Personally, I would go out with my handy dandy 70-300 and like playing a trombone, work the slide till I get it right. Should take less than a minute.

Reply
Page 1 of 4 next> last>>
If you want to reply, then register here. Registration is free and your account is created instantly, so you can post right away.
Main Photography Discussion
UglyHedgehog.com - Forum
Copyright 2011-2024 Ugly Hedgehog, Inc.