A question on EF-S vs EF exposure Levels

Greetings,

I’m looking for clarification on a point:

As I understand it, when mounting an EF Lens on a crop camera the amount of light gathered by the sensor is lessened by the smaller “bite” of the image circle, meaning I have to adjust either the ISO or Shutter speed to achieve the same exposure level when compared to a larger sensor.

But – I’m not clear on how this relates to EF-S lenses. Given that the image circle is smaller by design, does the exposure match to an EF mounted on a Larger sensor assuming the same Aperture, ISO, and focal length?

For example, if I have a 35mm F2.8 EF set on a full frame, will I get the same relative exposure on a scene with a 17–55 F2.8 EF-S set to 35mm with matching ISO/Shutter setting? (understanding I may have to frame the scene differently to account for the FOV)

In other words, does EF-S lens design negate the “loss” of exposure when translating EF lenses to Crops?

Thanking in advance for any enlightenment.

Up front, No.

That said, I am not sure the that one does need to adjust anything to get the same exposure level. The amount of light falling on the photosites in the area of the smaller sensor is not being reduced to such an extent.

Lastly, in your example, yes and no. Yes you'll get the same relative exposure if all else is equal. No you wont as the two lenses are quite different so it's unlikely that you would get identical behavior or measurements even if the "spec" is similar.

Honestly, I could be wrong on all counts here, but I think for practical application I am on the right track.

Thanks for the response. Even as I'm posting here, I'm starting to get the idea a bit clearer in my own mind - Matching the angle of view between the formats probably does more to equalize the exposure then anything else

KevinSch wrote in post #17046383
As I understand it, when mounting an EF Lens on a crop camera the amount of light gathered by the sensor is lessened by the smaller “bite” of the image circle, meaning I have to adjust either the ISO or Shutter speed to achieve the same exposure level when compared to a larger sensor.

You have totally misunderstood the "crop" issues.

You can use any lens on your camera and as long as you set the f-stop value, shutter speed, and ISO value to the same settings the exposures will all be the same (within very small tolerances) for a given scene with its lighting. It doesn't matter if you're using a so-called "full-frame" DSLR, a so-called "crop" DSLR, or a medium format film camera.

KevinSch wrote in post #17046383
But – I’m not clear on how this relates to EF-S lenses. Given that the image circle is smaller by design, does the exposure match to an EF mounted on a Larger sensor assuming the same Aperture, ISO, and focal length?

From an exposure point of view, there is absolutely no difference if you're using EF or EF-S lenses (or any third-party lenses that work on your camera).

KevinSch wrote in post #17046383
For example, if I have a 35mm F2.8 EF set on a full frame, will I get the same relative exposure on a scene with a 17–55 F2.8 EF-S set to 35mm with matching ISO/Shutter setting? (understanding I may have to frame the scene differently to account for the FOV)

In other words, does EF-S lens design negate the “loss” of exposure when translating EF lenses to Crops?

There is no "loss of exposure" to deal with. Period.

The f-stop setting is a ratio between focal length of the lens and the effective opening of the aperture. All of today's camera lenses are marked with their actual focal lengths regardless of the lens design.

Correct. Focal length, make or type of lens, or camera don't affect exposure. Only aperture, shutter speed, and ISO affect exposure for a given field of view. Even field of view can usually be ignored if the scene you're shooting is relatively even in exposure level.

Consider a handheld exposure meter, which happily does its job of providing the user with the correct f-stop, shutter speed, and ISO setting, without knowing anything about the camera or lens they're using.

^+1
Imagine taking a picture of a large grey card or for that matter a large evenly lit subject
The light gathered on each photosite will only depend on apperture and shutter speed
The crop factor = sensor size will only chagd field of veiw

Snafoo wrote in post #17046528
Correct. Focal length, make or type of lens, or camera don't affect exposure. Only aperture, shutter speed, and ISO affect exposure for a given field of view.

You can drop the "for a given field of view" part of that. The angle/field of view of a lens/camera combination does not affect exposure at all (assuming the whole target is equally lit, of course).

SkipD wrote in post #17046542
...(assuming the whole target is equally lit, of course).

That's what I meant when I bolded "for a given field of view". I edited my original post to clarify.

The image from an EF lens being projected onto an APS-C sensor would not suffer from vignetting, so would have a more uniform exposure across the frame when compared with that image on a full frame sensor.

But a full frame sensor gathers more photons in total, resulting in a more accurate representation of the scene, all other things being equal.

KevinSch wrote in post #17046383
Greetings,

I’m looking for clarification on a point:

As I understand it, when mounting an EF Lens on a crop camera the amount of light gathered by the sensor is lessened by the smaller “bite” of the image circle,

Less total light over a smaller area = same exposure. This if it this way, if the sensels are the same size in the two cases and only the image circle changes, then each sensel will get the same amount of light.

...

The same ISO/Aperture/shutter speed will give you the same subject exposure regardless of camera or lens.

The difference between FF and crop is that the performance of the bodies at each ISO setting is different. I.e. ISO 3200 will be noisy on a current generation crop, but significantly cleaner on a full frame body. That's because the full frame body gathers more light and thus has a higher signal to noise ratio at ISO 3200 than the crop.

Snafoo wrote in post #17046670
I'm not sure how your first comment is relevant, since vignetting is a function of the design and quality of a lens, not fundamental principles. As for your second comment, it makes no sense.

It is - kind of.

The full-frame sensor has a larger area. And this larger area will in total collect more photons. Since the number of photons collected at a given exposure level is a direct function of the sensor area.

So a full-frame sensor in the same technology as a smaller sensor will manage less noise. If the full-frame sensor has the same number of pixels as a smaller sensor, then each pixel will be larger and so collect a larger charge. So each individual pixel will have less noise.

If the full-frame sensor has the same size pixels as the small sensor, then each pixel will collect the same amount of light and will have the same amount of noise. But in this case the total sensor will have more pixels, so a full sensor print will have each individual pixel represent a smaller part of the print. So in this situation too, the full-frame sensor will give a print with less noise.

But as soon as the light level gets high enough, then even the small sensor will be able to collect enough photons that noise isn't really an issue. Which is a reason why mobile phones can manage excellent photos in good light.

But it's still quite correct. The camera with the smaller sensor has a smaller lens that will pick up a smaller amount of photons reflected from the subject. But since it needs to distribute these photons over a smaller sensor, it will still manage to project the same photon density onto the sensor. So it manages the same exposure level. And same size pixels in a small and large sensor will each see the same number of photons.

ejenner wrote in post #17046580
Less total light over a smaller area = same exposure. This if it this way, if the sensels are the same size in the two cases and only the image circle changes, then each sensel will get the same amount of light.

This isn't true. They get the same light projected onto both sensors, however the pixels are larger on the FF (in general). As a result you gather more light per pixel for a set aperture/shutter speed. Camera makers then calibrate their ISOs for the increase in light, so ISO 800 is the same regardless of camera. However, they calibrate the camera by increasing or decreasing the amount of amplification they have to do to the signal that comes off the pixel. The more amplification, the more noise gets amplified to reach the set ISO. With more light, comes less amplification (higher Signal to Noise ratio), and cleaner images at the same ISO.

Thanks for the detailed answers - I've also read another statement on DP Review - "ISO ensures that, if you expose a sensor to a given light intensity for a given amount of time, then you will get a certain brightness in your final (JPEG) image".
Along with the info in this thread, I think I can sort this out.

Hello 35mm F2 IS to my 60D