[Photobill]

I should know, but I don't.
When people talk of the deference between FF & APS-C regarding the f-stop they mostly cover the bokeh.
My question is:
When using my SMC Pentax 55mm F1.8 @ 1.8 on a APS-C camera does that sensor get approximately the same amount of light as using it on a FF @ the same f:stop? There is a lot of talk about loosing almost a full f: when using lenses on crop body's but can you please clarify if this is mostly about the bokeh or does it also play a part in exposure?
Sample
I use a light meter and it says shoot the scene @ f2.4 Would I get approximately the same expo with a FF & APS-C body? Or would I need to open up to f:2 or f:1.8 on the APS-C to get the equivalent exposure as on a FF.

As I said I should know this, but I do not 😞

Thanks to anyone that can set me straight. 🙌

[boriscleto]

If the light meter says f/2.4 you shoot at f/2.4. No difference between FF & APS-C. Aperture is aperture no matter what format the lens is designed for.

[GUB]

The easy way to think through this is to visualise the image circle and the two sensor sizes sitting on it. Then you can see that the image is exactly the same brightness for both sensors. The f stop is calculated by dividing the focal length by the diameter of the iris. Neither of these values change when changing sensor size so all exposure values remain unchanged.

[aslyfox]

the facts as I understand them:

1 I am easily confused

2 the sensor, whether large " full frame " or small " ASP-C " is an actual rectangle of a known size

3 the aperture allows a set amount of light [ brightness ] reflected from the target into the sensor and any light which falls outside of the sensor isn't seen

4 with the larger sensor, the reflected light hits a larger rectangle and you get more of the reflected light than with the smaller sensor,
in other words, you lose some of the top, bottom and sides using the smaller sensor

5 the brightness of reflected light doesn't change

isn't this what GUB is explaining ??

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Last edited by aslyfox; 08-16-2019 at 02:49 PM.

[From1980]

The easy way to think through this is to visualise the image circle and the two sensor sizes sitting on it. Then you can see that the image is exactly the same brightness for both sensors. The f stop is calculated by dividing the focal length by the diameter of the iris. Neither of these values change when changing sensor size so all exposure values remain unchanged.

A good, simple, and uncomplicated explanation. I've heard some confusion about this and other basic topics before and grew tired of people making a long drawn out discussion of it. Well done.

[swanlefitte]

On each pixel falls the same light. A k1 or k1 in in crop mode has the same light on the used pixels.

[boriscleto]

f-number - Wikipedia

A 100 mm focal length f/4 lens has an entrance pupil diameter of 25 mm. A 200 mm focal length f/4 lens has an entrance pupil diameter of 50 mm. The 200 mm lens's entrance pupil has four times the area of the 100 mm lens's entrance pupil, and thus collects four times as much light from each object in the lens's field of view. But compared to the 100 mm lens, the 200 mm lens projects an image of each object twice as high and twice as wide, covering four times the area, and so both lenses produce the same illuminance at the focal plane when imaging a scene of a given luminance.

[Apet-Sure]

When using my SMC Pentax 55mm F1.8 @ 1.8 on a APS-C camera does that sensor get approximately the same amount of light as using it on a FF @ the same f:stop?

An interesting question. I would think yes, and no. Yes, the exposure would be mostly the same on either camera assuming the light meter is set to center weighting. If the meter is averaging over the whole frame, the exposures could be different depending on where the light and dark areas are in 55mm FF coverage. Some of these might fall outside the coverage of the crop sensor, which would change the average. [After re-reading your post, I see you are using a separate light meter. If you meter your subject, then I think the f-stop would be the same on either camera.]


No, in that some of the light that the lens can transmit (assuming a FF capable lens) will spill off the side of the crop sensor. So technically the crop sensor is not 'getting' all the light that the lens is transmitting.

Somebody better versed in this might say my thinking is wrong. Oh well, I can live and learn.

[pschlute]

I use a light meter and it says shoot the scene @ f2.4 Would I get approximately the same expo with a FF & APS-C body?

What do the instructions for your light meter say ?

If you have mislaid them I can assist.

They do not say " the readings are for a specific format (aps-c/FF/MF). Light is light, aperture is aperture, format is irrelevant.

[stevebrot]

When using my SMC Pentax 55mm F1.8 @ 1.8 on a APS-C camera does that sensor get approximately the same amount of light as using it on a FF @ the same f:stop?

Yes...that is the magic of relative aperture (f-number). A given EV (shutter/relative aperture combination) will provide the same amount of light to the capture media regardless of the format or focal length.*

There is a lot of talk about loosing almost a full f: when using lenses on crop body's but can you please clarify if this is mostly about the bokeh or does it also play a part in exposure?

The talk is not founded on reality as far as pictorial photography goes. Put another way, when was the last time you saw either a change in brightness or bokeh when cropping in PP? Crop format has exactly the same effect on the image character as a crop in post.


Steve

* Since the f-number is based on the lens focal length at infinity focus, there is some loss of brightness with close focus due to magnification.

[Bob 256]

You will get exactly the same amount of light falling on either type of sensor. What may be different is that each pixel on the APS-C sensor could be slightly smaller than those in the full-frame sensor, and thus intercept less light, which could make the APS-C sensor less sensitive for a given image. It has nothing to do with f-stop, and is dependent on sensor type and pixel density (which can be different or identical when comparing APS-C to FF sensors).

That said, if you have the two sensor types and they have the same sensitivity, image brightness for equal f-stops will be identical for each. The same can be extended to different focal lengths as well (f4 for a 35mm lens will yield the same image brightness as f4 for a 50mm lens). One caveat here however, this applies to extended subjects (like a flat wall evenly lighted). Point objects, like stars or distant headlights will differ in brightness in proportion to the area of the aperture which will be different for the same f-stop for differing focal lengths. Thus, it's possible to get brighter star images by increasing the focal length used and maintaining the same f-stop (kind of goes counter to common sense). In this case, the aperture has to grow to maintain the same focal length, and its area increases making point objects brighter.

[ismaelg]

Hello,

Focal length and aperture are physical characteristics of the lens.
The difference is how much of the image is been recorded. I made this drawing to explain this a while ago to a local photo club. The full image is actually round and inverted but you get the idea.



By the way, if you haven't figured it out:
Luz = light
lente = lens
largo focal = focal length
APS-C = APS-C

Thanks,

[Ian Stuart Forsyth]

I should know, but I don't.
When people talk of the deference between FF & APS-C regarding the f-stop they mostly cover the bokeh.
My question is:
When using my SMC Pentax 55mm F1.8 @ 1.8 on a APS-C camera does that sensor get approximately the same amount of light as using it on a FF @ the same f:stop? There is a lot of talk about loosing almost a full f: when using lenses on crop body's but can you please clarify if this is mostly about the bokeh or does it also play a part in exposure?
Sample
I use a light meter and it says shoot the scene @ f2.4 Would I get approximately the same expo with a FF & APS-C body? Or would I need to open up to f:2 or f:1.8 on the APS-C to get the equivalent exposure as on a FF.

As I said I should know this, but I do not ��

Thanks to anyone that can set me straight. ��

You have to remember that how much light is gathered in an image and the size of the exposure are different things when comparing them to 2 different sensor sizes.

If we are using the same f-stop and shutter speed for both sensors then the exposure value will be the same but the size of the sensor recording the light is smaller so it captures less light. Exposure is really the amount of light pre unit of area value, so that we can expose 2 different size sensor the same but because we have different sizes of sensors less light will be captured on the smaller format.

Now we can set both cameras to capture the same amount of light if we select lenses with the same dia of iris for both cameras.

If we are using a 50mm @ f/2.8 lens on FF that iris is 50/2.8=17.86mm, to capture the same FOV as that 50mm lens on FF you would need to use 35 on your cropped body. Now if we use that lens at f/2.8 you are not using an iris of equal size as the one on your FF body 35/2.8= 12.5mm dia.

To use the same size of iris as that 50mm @f/2.8 on your cropped body you would need to use f/1.96 that 35lens divided by 1.95 = 17.9mm dia …… same size of iris same amount of light( but with a larger exposure value) will be projected onto the that smaller sensor.

[stevebrot]

If we are using the same f-stop and shutter speed for both sensors then the exposure value will be the same but the size of the sensor recording the light is smaller so it captures less light.

Not that it makes much difference from the standpoint of exposure (flux is the same).


Steve

(...these total light discussions are so predictable with the final word usually being had by the astro folk...)

[GUB]

A good, simple, and uncomplicated explanation. I've heard some confusion about this and other basic topics before and grew tired of people making a long drawn out discussion of it. Well done.

Yeeeep