Diffraction and Digital SLRs

[Don Hoey]

Stephen,

A very good question. My understanding is that it all down to the aperture of the lens itself. So if I am right, the converter will not have an additional effect. I don't have a TC so have not checked this out for myself.

Don

[Leif]

FWIW I agree with Don. Leif

[Don Hoey]

Originally Posted by Stephen Fox

What effect will a teleconverter have on diffraction? If the diffraction limit of an APS size sensor with 12Mpx is f11 what happens when the effective aperture is at f11? i.e. camera indicating f11 for exposure but using a 2X converter the primary lens will be f5.6.

Stephen,

Having just been through your gallery I cannot imagine you being much troubled by this. I notice you have only gone below f8 on your closeups. Always a f8 or wider on aircraft shots.

Just wondering if you are honing your reactions on Hoverflys as per Christines ( Sapphires ) pic. http://www.worldphotographyforum.com...ppuser=57&sl=s

Don

[Canis Vulpes]

Originally Posted by Don Hoey

Stephen,

Having just been through your gallery I cannot imagine you being much troubled by this. I notice you have only gone below f8 on your closeups. Always a f8 or wider on aircraft shots.

Just wondering if you are honing your reactions on Hoverflys as per Christines ( Sapphires ) pic. http://www.worldphotographyforum.com...ppuser=57&sl=s

Don

For close-ups I went smaller than f8 for depth and with aeroplanes my technique makes shutter speed king. If light allows a smaller aperture then the ISO gets lowered. f8 is about the cut off point to lower ISO. I have never been worried about f11 diffraction limit but I did know of it. Now I know why!!

When I get back home I'll go a test with TC and effective aperture (currently in Glasgow!)

[Saphire]

I have found the last couple of days that I get the best with my 75-300mm lens and any combination of extension tubes that f8 is best with or without flash. I have also found if I can get a 1000s speed its even better.

[Leif]

I thought I'd add this in case anyone is interested.

Anyway, I recently realised that my comment on smaller sensor not being so good for macro work is in fact incorrect.

It is often said that smaller sensors have more depth of field, but I have never understood this. Well DOF is a function of aperture and image magnification only. Reducing the aperture increases the DOF. Increase the image magnification reduces the DOF. That is why we usually use very small apertures for insects and other small objects.

That second point is I believe the reason why a smaller sensor has larger DOF. Consider a full frame camera and an APS frame camera both using the same lens at the same aperture. Let's assume that we are taking the same picture, thus the APS camera must be further away, and the image magnification will be lower. By that I mean that the ratio of the subject size to the image size, where image size is measured on the sensor, is less. And so DOF in the DX image is greater.

I would guess that the difference is about 1 stop. So a lens working at F16 on a DX sensor would give similar DOF to the same lens working at F22 on a full frame sensor. Again this assumes the 'same' image.

So my conclusion is that APS cameras are just as good as full frame ones for macro.

However, although for macro more DOF sounds good, many people like to use shallow DOF for artistic effect e.g. to highlight part of a scene. That I think is an important reason why some people do not like APS cameras.

Leif