pros/cons of two different lenses at the same focal length?

[dede95064]

Hi all,

I was wondering if there are significant drawbacks in having one lens act like a second one's focal length? For instance, say I have a 240mm lens but like the 360mm look so instead of buying a 360mm lens, I could use more bellows on the 240mm lens to achieve the 360mm look. Does that work as an acceptable substitute or would there be some problems with this method? I take it there would be possible problems, like vignetting on the lens you're trying to extend? Is it simply better just to go for the actual lens focal length as it was originally intended?

Thanks!

[Mark Woods]

Umm, the bellows are used for focusing the lens. This is focusing the lens. If one moves the camera closer to a subject, the bellows are extended & the image is larger -- not because the bellows are extended, but the bellows need to be extended because the camera is now closer. What you speak of is a zoom lens. All of that said, you might approximate the effect of a longer lens by cropping the shot so you're using less of the negative. It's a close approximation of a different focal length (longer), but not exact.

I hope this helps.

[Two23]

The magnification will be the same but the perspective will be different.


Kent in SD

[Mark Woods]

Yes

[Bob Salomon]

No, it would still be a 240 if you want it to look like a 300 you will need a 300.

[Eric Woodbury]

Use a pinhole. Focal length is wherever you use it. Not always perfectly sized for lowest diffraction, but close enough. I suppose you could have multiples holes in a pinhole plate and pick the size you want by covering/uncovering the various pinholes as a means of optimization.

[John Kasaian]

If I wanted both a 240 and a 360 but could only afford one lens, I'd split the difference and get a 300.
It would be close!
Or look for a convertible lens. IIRC there was a convertible Symmar that might put you in the focal length ball park.

[Vaughn]

Framing a head and shoulders portrait with the 240mm, because of the closeness of the camera to the subject, you will get more of a perspective shift (foreshortening) than with a head and shoulder image with a 360mm. So the look/feel will be quite different. If you position the camera (w/240mm) the same distance from the subject as one would position a camera with a 360mm for a H&S image, then cropped the image to a H&S, you would have the same look, but as Two23 mentioned, the image will be smaller.

So this route will depend on the size of your format and how small of an image are you willing to end up with.

[Bernice Loui]

Short answer, no.

Bellows length is much about lens distance to image recorder (film or _) required to focus a given lens to the object to be imaged.

Note examples number one & two from this web page.
https://fivedayfilm.com/guide-to-cinematic-shots/


Bernice

[Mark Sawyer]

Technically, it would work, but the image at 360mm would be waaay out of focus. If you were at a very small aperture, that would bring it back into near-focus somewhat-if-you-aren't-too-fussy, and the angle-of-view would be that of a 360mm.

So yeah, it would work. Very, very badly.