11 May 2011

Some words about "bad" lenses (soft, backfocus, frontfocus) ...

I am constantly asked about this and this is frequently on forums. Is this true? Are lenses really bad?
Yes, you can have bad lens, but we will talk about "bad" lens. What do I mean?
There are bad copies of lenses, but if my information is good, we can talk in range from 3% to 7%.

Why people think they have a lens with backfocus or frontfocus?
Usually it goes in two steps:
1. Photographer notices that photo isn't sharp at focus point, but a little before (or after). This can be more obviously with open apertures (Especially very open like f1.8 ...).
2. Photographer makes a test with backfocus/frontfocus chart with camera on tripod.
And results can really show backfocusing/frontfocusing.

Why is this so often?
We can say that manufacturer will consider a camera and/or lens to be in their specifications if its + or - 5 units from perfect. We can assume they reached this number because anything within + or - 5 units will be within the depth of field of a wide aperture lens.

Let’s look at example:
I have a camera that is -3 units from perfect, and a lens that is +3 units from perfect. For both manufacturer said that are good. Both examples are in range, but aren't perfect. However, my combination will be perfect. My combination will focus very accurate.

Let’s look at second example:
I have a camera that is -3 units from perfect, and a lens that is -4 units from perfect. Again, manufacturer claims both are good. But my combination will focus -7 units from focus point (frontfocusing). If my lens is wide open, then my focus point won't be sharp!!

This is the reason why service guys need a combination of lens and body if they want to help you. But be careful - do you know specifications of your other lenses in a bag. Take them all (with body) to calibration!!
Prepare money ...

And because you are so patient reader, you deserve a trick. Many newer cameras have a lens micro calibration feature. Many people don't take advantage of this feature. But after micro calibration, camera will remember lens and will automatically compensate the difference.

After you get a lens, check it. You don't need a special test, just lay your gear on table, put a pencil on it and focus on a pencil (remember where you focus). Check on your computer at 100%. If you see some issue, download a test chart and make another test with tripod.
If lens has backfocus or frontfocus, read a camera manual and try micro adjustment. If you don't know how, ask your guru or go to service.

And don't waste a lot of time if something isn't perfect. If your photos are good, nobody won't notice a small issue. Call your girlfriend and go to nature (you can take photo gear with you :-)) and have a great time :-)


Neitz said...

hmmm, zanimivo. Nisem vedel da to tudi obstaja

Iztok Grilc said...

Pa ne boš verjel koliko ljudi se pritožuje nad tem ...