Is F8 really F8?

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Is F8 really F8?

Postby abennettphotography » Sat 02 Jan 2021, 13:16

I think I know the answer to this but but just checking my thinking....

Take a F4 lens (or one opened up to f4). Add a 2X TC. The lens will now act like an F8 in terms of aperture and DoF. Optically, most (all?) lenses are at their optimum at F8 (or thereabouts). But, the lens hasn't changed. In other words it acts like a F8 but optically it is still an F4 (disregarding the effect of the TC). Is this correct?
Mike Farley
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Joined: Tue 11 Sep 2012, 16:38

Re: Is F8 really F8?

Postby Mike Farley » Sat 02 Jan 2021, 18:59

abennettphotography wrote: ......most (all?) lenses are at their optimum at F8 (or thereabouts).

The answer to that is complicated and what follows is a simplification of a complex topic. Lenses typically achieve their best performance in the centre of the image around two stops down from their maximum aperture. However, performance in the corners is always less so it is usually better to stop down a bit further if edge to edge sharpness is required. It works both ways. Sometimes it can be desirable for an image to be softer at its edges to help direct the eye towards the subject. Another factor is the quality of the lens. One reason that expensive ones cost what they do is that they are designed to perform well at their widest apertures.

Most lenses will indeed be optimal around f/8 which makes it a useful setting in many scenarios. If your speciality is shooting brick walls, going much beyond that risks reducing image quality due to diffraction. The point at which that occurs will depend on sensor size, the larger the better. With the smaller sensor in MFT (Micro Four Thirds) cameras, it is starting to happen by f/8. With a full frame sensor, it will start at around f/11. In the real world, diffraction is less of an issue as much of any degradation can be recovered with good sharpening technique.

abennettphotography wrote:..... the lens hasn't changed.

The lens has changed, optically at least. As an example, a 300 f/4 lens with a 2x TC has transformed to a 600 f/8. It will only become f/4 again once the TC is removed. If the lens has an aperture ring, the indicated f-stop will be faster than the actual value. When the lens and TC have electrical contacts to communicate with the camera, often it displays the working aperture. Note that any optical imperfections in the lens, such as spherical aberration, will be amplified by the converter. For that reason, it is usually best to use a TC made by the same manufacturer as the lens since they will be designed to work together with the least compromise.

The optimum aperture in any given situation for a lens and TC combination will be a balance between several factors. Optical considerations aside, a smaller aperture requires a slower shutter speed and/or increased ISO. Depending on circumstances, both can have an undesirable effect on the final image.

Now to blow your mind. Focal length reducers are designed to restore the stated focal length of a lens designed for a larger sensor than is in the camera. The most common application is for older lenses designed for 35 mm film systems which are instead used on a crop sensor. For example, a 50 mm lens where there is a 1.5 crop factor would act like 75. The reducer works as an inverse teleconverter and restores the original focal length. Just as a teleconverter reduces the maximum aperture, a reducer increases it. The same laws of physics apply only in reverse. If that 50 mm lens has a maximum aperture of f/2, with the reducer it would become more like f1.7.

There is more information in this white paper on focal length reducers from Metabones: ... 0Paper.pdf

Mike Farley
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