Apeture size

[Goldcoin79]

If I am working out the size of the apeture do I use the actual focal length or the 35mm equivalent value eg for a 50mm focal length with an apeture set at f5 on a camera with a 1.6 crop factor is the calculation 50/5 or 1.6 X 50/5

 

[kundalini]

The focal length of the lens is what's stamped on the barrel. A 50mm lens is a 50mm lens. The difference seen is in the Field of View (FoV), not the focal length.

Aperture has no bearing. Use the appropriate aperture for your shot / vision of the scene.

 

[KmH]

Use the actual focal length.

When you see the term f/4, the f = the focal length of the lens.

So a 50 mm lens, or zoom lens set to 50 mm, set to f/4 has a 12.5 mm diameter lens aperture (50 mm / 4 = 12.5 mm).
A 35 mm lens, or zoom lens set to 35 mm, set to f/4 has a 8.75 mm diameter lens aperture.

When considering Depth-Of-Field (DoF), then the image sensor crop factor does have a bearing, and online DoF calculators make you specify the crop factor in some way. Many just have you select the camera make/model you use.
Understanding Depth of Field in Photography

 

[Goldcoin79]

Thanks KmH that is the answer I was looking for. When you are talking about the dof calculator are you saying that they calculate the dof by using the crop factor to first calculate the 35mm equivelant?

 

[Helen B]

No. With most calculators you use the actual focal length of the lens (and the marked aperture) and specify the format separately.

If you are doing a DoF comparison between formats for similar fields of view and distance then you can use the crop factor on both the focal length and the f-number. For example if the crop factor is 2, a 25 mm lens at f/2 on the crop camera is equivalent in FoV and DoF to a 50 mm lens at f/4 on a full frame 35 mm camera.