Question about Verito Lenses

[LFLarry]

I recently acquired a Verito F4 soft focus lens, and I just learned that you can remove the front element, and this makes the focal length a little longer. Will the aperture remain F4 with the front element removed with the longer focal length? If not, how do you calculate the effective aperture?

Thanks

Larry

[xkaes]

Yes, the focal length changes, and yes the f-stop changes. How much depends on the structure of the lens. For example, if removing the front glass cuts the focal length in half, the f-stop doubles. If you can figure out the new focal length after removing the front glass, you can compute the new f-stop. One way would be to remove the front glass, focus on infinity, and measure the bellows length.

[djdister]

Removing the front element increases the effective focal length, as shown in the table at the bottom of this Verito catalog page (rear focus). Given the new effective focal length, I'm sure there is a quick formula to convert the marked f stop. Someone here will know that.

[LFLarry]

Removing the front element increases the effective focal length, as shown in the table at the bottom of this Verito catalog page (rear focus). Given the new effective focal length, I'm sure there is a quick formula to convert the marked f stop. Someone here will know that.

Thanks for the info. Very helpful.

Yes, hopefully, someone here will know how to calculate the effective aperture.

Thanks again.

[Kiwi7475]

Just scale by the ratio of the new focal length to the nominal length … you don’t say which Verito you have but say you have the 8 3/4” that turns into a 14”… then F4 * 14 / 8.75 = F6.4. Same factor of 14/8.75 applies if you use a smaller aperture….

[LFLarry]

Just scale by the ratio of the new focal length to the nominal length … you don’t say which Verito you have but say you have the 8 3/4” that turns into a 14”… then F4 * 14 / 8.75 = F6.4. Same factor of 14/8.75 applies if you use a smaller aperture….

Excellent, thank you. Exactly what I needed to do the math. Much appreciated.

[Tim Meisburger]

The f‑number equals the focal length divided by the entrance pupil diameter. For example, if you have a 100mm lens (i.e. a focal length of 100mm when focused at infinity), and its widest opening is 25mm, then that is an f4 lens (100/25=4). When you extend your bellows beyond 100mm, the actual f stop increases, which is why you need to use exposure compensation with macro shots. For example, if you extend your lens to 200mm, that is the new focal length, so 200/25=f8. So your 100mm f4 lens is an f8 lens at 200mm.

To calculate any lens, focus to infinity and measure the distance from the ground glass to the center of the lens and that is your focal length (more or less). Then measure the entrance pupil of the lens at its greatest opening, and divide the first number by the second number to get f-stop at infinity. So, take off the front element of your verito, focus at infinity, then measure, and you will know your new focal length.

Hope that is useful.

[neutron450]

Could someone elaborate a bit on methods for measuring the "entrance pupil" of a lens?

[xkaes]

It's actually not the "entrance pupil" that you want to measure. It's the widest diameter that the lens (and shutter, if there is one) provides that is nothing but clear glass all the way through. Simply lay a tape measure across the front on the lens and measure the widest diameter that is clear glass -- looking straight through the lens.

[Tim Meisburger]

In an optical system, the entrance pupil is the optical image of the physical aperture stop, as 'seen' through the front (the object side) of the lens system. In photography, the size of the entrance pupil (rather than the size of the physical aperture stop) is used to calibrate the opening and closing of the diaphragm aperture. (Wikipedia) But yes, that is how I measure it, as what you are measuring is the projected image. If you take off the lens and measure the diaphragm of the shutter instead, that may be different.