Prime vs. Zoom speed (Page 2): You are correct re T stops but for what the op was asking was photographic lenses, T-stops are measured for video lenses.

RichardSM Loc: Back in Texas

Dik wrote:

Wrong wdross,
T-stop lenses transmit the same amount of light at the same setting, but f/stop lenses do not.
I confirmed this myself shooting a Canon 24-105L at 40mm f/4, and a Voightlander Ultron 40mm f/2 at f/4.
There was a little more than a stop difference. I had to set the prime to f/5.6+ to match the zoom .

You are correct re T stops but for what the op was asking was photographic lenses, T-stops are measured for video lenses.

Dik

OP was wondering why his zoom required more exposure than his prime at the same f/stop. f/stop vs T-stop for same lens construction (google Cine Lens) shows this. Different lenses at the same f/stop deliver different amounts of light to the camera. In-camera metering automatically compensates for this effect, so it does not become apparent to most photographers.

CLF Loc: Raleigh, NC

Dik wrote:

OP was wondering why his zoom required more exposure than his prime at the same f/stop. f/stop vs T-stop for same lens construction (google Cine Lens) shows this. Different lenses at the same f/stop deliver different amounts of light to the camera. In-camera metering automatically compensates for this effect, so it does not become apparent to most photographers.

Dik, First off, thanks to ALL for the discussion. Dik, you have stated technically which I was unable too do. Comparing those two lenses at 300mm I was experiencing that the 300mm passed more light to the sensor than the Zoom. Both lenses are top shelf Canon L lenses so the quality of both is fantastic and when compared from that stand point there is little to no difference. Being able to gain an f stop using the 300mm prime does help in my daily pursuit of moving subjects which are birds and mammals. Yes, one f stop is not much but it is an edge to take advantage of. My $.02.

Greg

Dik

Thanks Greg, Glad to confirm your findings with the tech explanation.
I have 60 year's experience with f/stops, but only realized this about a year ago.

dsmeltz Loc: Philadelphia

Dik wrote:

OP was wondering why his zoom required more exposure than his prime at the same f/stop. f/stop vs T-stop for same lens construction (google Cine Lens) shows this. Different lenses at the same f/stop deliver different amounts of light to the camera. In-camera metering automatically compensates for this effect, so it does not become apparent to most photographers.

I do not use DxOMark for much since I am more interested in actual field performance, but T-stops are one thing I look at there when comparing lenses. However, I do not remember seeing that large a variation on a lens.

They show the 300 @ f2.8 with T=3.2 and the 100-400 at 200mm f5 with T=5.6.

When the OP ran the test, what f stop was used on each lens? He mentions both were set to the same length but did not mention the f.

CLF Loc: Raleigh, NC

Dik wrote:

Thanks Greg, Glad to confirm your findings with the tech explanation.
I have 60 year's experience with f/stops, but only realized this about a year ago.

Thanks again, Dik.

Greg

Dik

dsmeltz wrote:

I do not use DxOMark for much since I am more interested in actual field performance, but T-stops are one thing I look at there when comparing lenses. However, I do not remember seeing that large a variation on a lens.

They show the 300 @ f2.8 with T=3.2 and the 100-400 at 200mm f5 with T=5.6.

When the OP ran the test, what f stop was used on each lens? He mentions both were set to the same length but did not mention the f.

I too am a little surprised at the 1-2 stop difference reported. Perhaps he meant one or two clicks on the adjustment wheel (1/2 or 1/3 f/stop per click).

wdross Loc: Castle Rock, Colorado

Dik wrote:

I too am a little surprised at the 1-2 stop difference reported. Perhaps he meant one or two clicks on the adjustment wheel (1/2 or 1/3 f/stop per click).

Dik, the possible reason for not seeing a significant difference in exposure between primes and zooms for my Olympus may be because Olympus uses lots of elements in both the primes and zooms (e.g., look that the new f1.2 primes). Personally, I feel anything a 1/3 of a stop or less is a somewhat insignificant except for special circumstances.

Dik

Could be. I tested using the Voightlander Ultron 40 f/2 as prime because it has relatively few elements, compared to the 24-105L.

speters Loc: Grangeville/Idaho

CLF wrote:

I have a question concerning a difference I have n... (show quote)

It depends on the lens in particular, but as a rule, primes are often/mostly faster than zooms - just in part of how they are designed/constructed. But as I said, it depends on the lenses in particular. I have a zoom for example, that is pretty much the sharpest lens that I own ( and I do own a lot of primes), its an Angenieux 15x zoom lens. It is /1.2 throughout, so its very fast, I can shoot at candle light and get nice bright shots. It handles low light better (and with better results) than any of the "faster" primes that I do own. So in general, primes do let in more light, but there are exceptions!

Dik

speters wrote:

It depends on the lens in particular, but as a rul... (show quote)

Sounds like a movie camera lens. Is it marked in f/stops or T-stops?
I could not find a 15x f/1.2 but I found a 12x f/2.5, it's Transmission # is 2.8, only a 20% difference, so not a lot of improvement to be had with a "perfect" lens of any type.
T-stop numbers for the same lens are always higher. How much depends on how much the lens absorbs and scatters light. More elements and air to glass surfaces, absorb and scatter more light, so your 15x Angenieux, if marked f/1.2, will not transmit as much light as a high quality, prime lens with many fewer elements, assuming similar coatings and glass type. Absorbing only 20% is quite an accomplishment, for such a busy lens.

Dik

The OP's zoom has three times as many elements as his prime - 21 vs 7.

Dik

My Canon 24-105L has 3x as many elements as my Ultron 40 f/2, 18 vs 6.
I'm gonna test again.

Architect1776 Loc: In my mind

wdross wrote:

I have set my primes and zooms using a light meter to the f-stop indicator by the meter and have gotten identical exposures. I have also shot with a prime and changed to a zoom and used the prime's exposure to get the same exposure with the zoom. There may be something wrong with the mechanics of one of your lenses.

There are small variations with f stop but not significant.
That is why a hand held meter for all your lenses if it says 1/500 at f5.6 it does not care what lens you have on the camera. That is the setting to use for correct exposure.
T stops are more accurate and actually measured for cine lenses. That is on reason for the extra cost as they are matched and carefully made that way. There are other reasons too.
In my short experience whether a wide lens, normal telephoto or zoom I found a meter setting worked for all of them as read.
If you are getting 1 stop or more issues then there is something wrong with one of the lenses.
I just tried it with a 100-400L and a 100 L prime and there was no difference in the reading.

billnikon Loc: Pennsylvania/Ohio/Florida/Maui/Oregon/Vermont

CLF wrote:

I have a question concerning a difference I have n... (show quote)

This is a interesting question coming from someone with over 13,000 posts. And yes, generally speaking a fast lens lets in more light. It does not matter the number of elements, it's the size of the front and rear elements that makes the difference.