f stop and crop sensors

[skybushman]

So, while neither the focal length nor the aperture diameter are affected by the body the camera is on, the equivalent focal length is affected, and since the equivalent focal length is affected, the equivalent f-stop is also affected. So, if you multiply the focal length by the crop factor to get the equivalent focal length, then you must also multiply the f-stop by the crop factor as well to get the equivalent f-stop.

Who does this much math when composing a shot? - your post is very informative but to the novice (me) I guess my first question is am I supposed to be crunching these numbers before composing a shot? In most instances my subject would have moved on.

 

[Gerry Winterbourne]

So, while neither the focal length nor the aperture diameter are affected by the body the camera is on, the equivalent focal length is affected, and since the equivalent focal length is affected, the equivalent f-stop is also affected. So, if you multiply the focal length by the crop factor to get the equivalent focal length, then you must also multiply the f-stop by the crop factor as well to get the equivalent f-stop.

Who does this much math when composing a shot?

No one, I hope.

- your post is very informative but to the novice (me) I guess my first question is am I supposed to be crunching these numbers before composing a shot?

The point about information like this is that it helps you understand what happens. Then you think about that and apply the principles to the sort of pictures you take, with the camera and lens(es) you use. It's usually a good idea to take a series of shots at leisure in the comfort of your own home or garden.

That way you can be prepared for situations you are likely to encounter so that ...

In most instances my subject would have moved on.

... you know what settings to apply as you approach your subject and before it has a chance to move on.

 

[PhotoTeach2]

• That's right. The size of the sensor does NOT alter the size of the aperture... and, for exposure purposes , f/8 remains f/8 whatever size of lens/sensor/camera is in use.
• But for Depth of Field purposes , the same aperture number does NOT have the same effect on all formats. Smaller sensors exhibit more Depth of Field when Field of View and final image sizes are matched. As an example, the gain in DoF between Full Frame and APS is close to one stop's worth.... f/8 on APS being DoF equivelant to f/11 on Full Frame.

So if I have a fast lens that takes good exposures in low light on a FF sensor, that same lens is going to take essentially the same good exposure in low light, AT THE SAME SETTINGS, on a crop sensor.

Yes !!!

But I will have a narrower angle of view, and a wider depth of field.

Yes !!!

Is that correct?

Yes !!!

 

[skybushman]

Thanks Gerry!

 

[bjn70]

My question is: does that effect the f/ number as well? I think it doesn't, but I want to be sure. So for the 2 lenses mentioned above, the f/ will remain f/4 and f/1.4 and widest aperture, correct?

The aperture stays the same with respect to exposure. Set ISO and aperture the same on both bodies, point to a plain wall and you should get the same shutter speed from both bodies.

But there is a secondary effect- the depth of field that you get from one camera will be different from the other at the same aperture. So people will tell you that the crop factor applies to the f-number as well. Not for exposure but for depth of field purposes. If you are shooting subjects where you want to isolate the subject from the background, things such as portraits, then it is a little more difficult to do with the crop sensor body than the full frame body. If you are shooting subjects where it is less critical such as landscapes, then there really isn't much affect.

(edited to say that I didn't realize this was a 7 year old discussion)

 

[Azathothh]

Newbie question here. I have 2 Canon lenses, EF 24-105mm f/4L IS USM, and EF 35mm f/1.4L USM. Usually they are attached to my 5D Mk1.

I am getting a 7D today. I know it's a crop sensor with a 1.6x mag factor, so my lenses are really 38-168mm and 56mm.

My question is: does that effect the f/ number as well? I think it doesn't, but I want to be sure. So for the 2 lenses mentioned above, the f/ will remain f/4 and f/1.4 and widest aperture, correct?

Also, are there any other considerations I should be aware of in taking EF lenses and putting them on crop sensor (EF-S) bodies?

Thanks in advance for any and all who care to take a run at this.

No. Only field of view changes. But that also means it will bring consequences for the depth of field for photos with same framing taken with different sensor sizes. You will stay more far away from your subject when using the 7D which means a bigger depth of field, a less blured background.