general hasselblad lens question

[frdchang]

i noticed a scratch on the rear element of a 50mm hasselblad chrome

lens... its superficial because you can't really feel it... but its

not invisible... its pretty noticeable.

 

under what shooting conditions (apeture, and focusing distance) will

the rear element scratch kinda show? (if at all.. i'm hoping the

scratch won't impeded optical performace)

 

thanks,

fred

[richardcook]

My understanding is that this is mostly a good news thing. You can do a lot of damage to a lens or even an astronomical mirror without having a significant effect. Some years ago a nut case used a pistol to shoot at a major telescope mirror (Kitt Peak, as I recall, but don't hold me to that) which resulted in a rather large ding in the optical surface and a great deal of consternation among the astronomers. Everyone was surprised to find, however, that the effect of the damage on the image was quite small; the surface area of the mirror is so large that the damaged area's contribution did not spoil the quality. Really large astronomical lenses often contain internal bubbles. The enormous effort it takes to get high precision on these lenses would make you think that such a defect would ruin the image but the effect is actually quite small.

 

This is a neat part of optics. Every point on the surface of a lens contributes something to each point point in the image plane. We all understand that the lens focuses all the incoming light from a point in the scene to a correpsonding point on the image plane. What we tend to foget, however, is that, in a perfect lens, EVERY point on the lens surface bends the incoming light rays from every part of the scene to form the image so that EVERY point on the lens contributes to EVERY point of the resulting image.

 

This has both good and bad consequences. Because the rest of the lens helps form the image everywhere on the image plane, the images produced from a scratched lens usually look pretty good -- the rest of the lens makes up for the lost area. You can have quite large defects in lenses and still have recognizable images because the all points on the lens surface participate in creating each point of the image. It's not all skittles and beer, though: the same effect also distributes the consequences of the defect over the entire image plane. The effect is that a defect makes the ENTIRE image worse -- but by a very small amount. People with cataracts know this well: a discrete defect in their lens does not blur a part of the image they see but, instead, contributes to degradation of the WHOLE image.

 

Interestingly, a scratch probably degrades image quality less than smudges or fungus: the surface area of the lens that is involved determines the magnitude of the effect and a scratch tends to involve small amount of surface.

[jay_.]

Coating scratches on a rear element are much more serious than those on the front element. The front element marks reduce contrast and promote flare (the more scratches the worse it gets) but don't affect the resolution. Scratches on the rear element can affect the sharpness, particularly the wider the angle of coverage. If you already have this lens and aren't noticing any problems, great. If you're contemplating buying this lens, test it thoroughly and if it meets your approval then get the price reduced significantly as such a scratch will lower the resale value.

[josef_geisler]

I have a 80 mm planar with a small scratsch on the back lense. Can not detect any problems. The lense still performes like new. Good luck with your lense. Joe

[c-p-j]

What's the effect of a small scratch you can see but not feel?

 

None!