lens brightness

tballphoto · Nov 12, 2021

Is it possible for a lens to go darker over time, or to just not give the same light through put as it ages?

I have a lens that used to be pretty good when i got it used, but it seems to be darker as it gets older. I mean at least the image in the view finder is darker then it used to be.

ic-racer · Nov 12, 2021

I have seen aperture blades that stick and leaf shutters that fail to open all the way.

tballphoto · Nov 12, 2021

its an aftermarket fd mount lens, telezoom 100 to 300. Wide open, fully extended it almost blacks out in certain situations, cant even see the view finder data. But when i toss my other lenses on, its getting enough light to see.

Example, if i put the good ones to a certain aperture and focal length, then put the screwy one one the camera to the same settings i may merely get extremely dark "i think thats big foot.." in the view finder to complete black out

beemermark · Nov 12, 2021

Not really. Even if they did the pupil in your eye would open wider to compensate. More likely answer (got I forget the scientific answer) is your pupil just doesn't open as wide as it used to. Check out astronomy tips on viewing. a 10 year can see more than stars than a 20 year who can see more stars than a 30 years old. And by the time you get into your 50's forget it -. everybody eyes are different but they follow the same progression.

MattKing · Nov 12, 2021

Make sure that you aren't using the lens stopped down and in stop down metering mode.

AgX · Nov 13, 2021

Some optical (radioactive) glass changes over time yielding higher optical density.

Petrochemist · Nov 13, 2021

Yes, there are several factors that can reduce the transmission of glass as it ages.
The most common is internal haze or dirt, both of which tend to decrease contrast as well but don't normally prevent the lens from forming images. Cleaning the lens internally can be relatively simple or can be extremely complicated depending on lens design.
Some lenses (typically those employing thorium glasses) can develop a amber colour within the glass. exposing the glass to sunlight/UV for several days usually reverses this.

At the edges of visual wavelengths, some glasses are naturally less transmitting, but I don't think this is a factor that changes significantly with time. This is most common at the UV end of the spectrum (the border between visual & UV depends on the viewer)

neilt3 · Nov 13, 2021

Some focus screens are better than others for telephoto lenses .
What screen is in your camera ?
Is it the original one or has it been changed ?

tballphoto · Nov 14, 2021

canon ftb original view screen.

Its just the lens is odd now. changing aperture doesnt make a difference, and changing the focal length doesnt make a difference in how dark the image in the view finder is.

RalphLambrecht · Nov 14, 2021

Bezoar said:
Is it possible for a lens to go darker over time, or to just not give the same light through put as it ages?

I have a lens that used to be pretty good when i got it used, but it seems to be darker as it gets older. I mean at least the image in the view finder is darker then it used to be.

Not unless something went wrong with the lens mechanics.

Alan Edward Klein · Nov 14, 2021

Have you cleaned the viewfinder and mirror?

tballphoto · Nov 14, 2021

Alan Edward Klein said:
Have you cleaned the viewfinder and mirror?

mirror is nice and shiny, and this was the first time i have taken that particular lens off the camera in perhaps 2 years.

All i can do is clean the outside of the view finder as i dont have the tooling to remove the top cover... And if i screw that up, it will be at least 170$ to have it fixed by a professional... MORE if i damage any of the parts.

250swb · Nov 15, 2021

tballphoto said:
its an aftermarket fd mount lens, telezoom 100 to 300. Wide open, fully extended it almost blacks out in certain situations, cant even see the view finder data. But when i toss my other lenses on, its getting enough light to see.

Example, if i put the good ones to a certain aperture and focal length, then put the screwy one one the camera to the same settings i may merely get extremely dark "i think thats big foot.." in the view finder to complete black out

When a telephoto lens is at it's longest focal length the maximum aperture will be limited to maybe f/6, when it is at it's widest the maximum aperture will increase, maybe f/4, which will transmit far more light. I imagine you are seeing this effect.

lens brightness

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