Prime Lenses

[Sailor Dave]

I have recently been doing some still-life photographs. I noticed that using a fixed aperture of f/5.6 for the given lighting condition my 90 mm Prime for a correct exposure was 1/10 sec with ISO 200. My 50 mm Prime at the same aperture and ISO for a correct exposure shows 1/5 sec. Am I right in assuming that it is because it is almost half of the 90mm focal length? that it is a slower shutter speed?
I also used a 23 mm Prime and the 23 returns the same as the 90mm 1/10 Both the 90 mm and 50 mm were Fuji primes f/2 the 23 mm was a Viltrox f /1.4Many thanks in advance.

[DanK]

I have recently been doing some still-life photographs. I noticed that using a fixed aperture of f/5.6 for the given lighting condition my 90 mm Prime for a correct exposure was 1/10 sec with ISO 200. My 50 mm Prime at the same aperture and ISO for a correct exposure shows 1/5 sec. Am I right in assuming that it is because it is almost half of the 90mm focal length? that it is a slower shutter speed?
I also used a 23 mm Prime and the 23 returns the same as the 90mm 1/10 Both the 90 mm and 50 mm were Fuji primes f/2 the 23 mm was a Viltrox f /1.4Many thanks in advance.

Focal length should have no effect because f-stops take focal length into account. F-stop refers to the relationship between aperture width and focal length, but that ratio only approximates how much light is transmitted. Transmission is affected by other aspects of lens design. The index of actual transmission is called "T-stop" and is in my experience never provided with lenses.

I'm surprised, however, that the difference is a full stop. Try setting your camera for the smallest aperture increment it will do, which is often 1/3 stop. Then keep the shutter speed the same and use aperture to see how different the estimated exposure is.

[Manfred M]

The focal length could have an (indirect) impact as a shorter focal length will have a wider field of view. That means a longer lens (like the 90mm) will likely include less background than the shorter 50mm lens. If your background is darker or lighter than your subject, the camera's metering system will get a different exposure requirement and your exposure could be quite different.

That is the drawback of the reflective metering system used by your camera. It "assumes" an average scene and adjusts exposure accordingly. If your background is darker or brighter than your subject, you could be seeing this in your exposure.

[Geoff F]

It can be difficult to do direct comparisons between different lenses. For example, I have an older Sigma 180 macro lens (with 72 mm diameter lens) and the newer larger model (with 86 mm diameter and stabilisation). There is a consistent variation with the newer lens requiring around 2 stops extra difference to achieve the same image brightness.

So I use the new 86 mm lens on sunny summer days but revert to the older lens for dull conditions.

[DanK]

To see whether the issue is lens specifications or the background issue Manfred described, do the test I suggested when focusing on something uniform, like wall that is blank for the full angle of view of the shortest lens you test.

[Manfred M]

One thing to remember is that the f-stop is simply a calculated value: the focal length / Diameter of the entrance pupil. It does not take into account the amount of light transmitted by the lens, which is primarily related to the number / thickness of the lens elements, well as the quality of the anti-reflective coatings and the number of air / glass interfaces. Two different lenses with the same aperture can (and will) transmit different amounts of light.

If you look at lenses designed for cinematography, the lenses are marked with T-stops, where this variability in light transmission is taken into account. The T-stop number will always be larger than the corresponding f-stop for the lens.

What surprises me a little is that 50mm lenses are usually the ones with the fewest number of lens elements (and air / glass interfaces) so often transmit light quite well.

[Sailor Dave]

Many thanks, for your reply I will try what you have said.

[Sailor Dave]

Many thanks for your reply.

[tclune]

One thing to remember is that the f-stop is simply a calculated value: the focal length / Diameter of the entrance pupil. It does not take into account the amount of light transmitted by the lens, which is primarily related to the number / thickness of the lens elements, well as the quality of the anti-reflective coatings and the number of air / glass interfaces. Two different lenses with the same aperture can (and will) transmit different amounts of light.

While what you say is true, the variation is not very large. If it were too big, a hand-held light meter would not be useful. Your point about metering a different scene with the two different focal lengths seems more likely to be the source of the large variation the OP is seeing I would guess.

[Manfred M]

While what you say is true, the variation is not very large. If it were too big, a hand-held light meter would not be useful. Your point about metering a different scene with the two different focal lengths seems more likely to be the source of the large variation the OP is seeing I would guess.

Indeed. I have seen reading differences of greater than 1-stop between different camera models / lenses when using a hand-held meter flash meter (Sekonic L-358), in a studio setting. I had assumed that at least part of the difference was due to differences in the way that ISO is calculated by different camera manufacturers.