[stevebrot]

Ok...This totally took me by surprise.

With the Pentax-FA 35/2 on my K10D, there is a half-stop difference in exposure at infinity vs. closest focus.

I tried it with several other lenses and all except the DA 18-55 kit showed the same pattern...1/2 to 1 stop more exposure at infinity than at closest focus.

How do these lenses differ from the kit?

• All FF, made for 35mm film
• Rear element moves when focusing

At first I thought it was my Katzeye screen...nope! I swapped out the Katzeye for the Pentax LF-80 and got the same results.

I repeated the test pointing the camera at a blank wall. Same results.

Any ideas what is happening? Let me know your thoughts!

(I have a notion, but want to hear a few other opinions before I stick my neck out.)

Steve

---

Last edited by stevebrot; 01-25-2009 at 08:56 AM. Reason: Changed title and text for accuracy and clarity

[axl]

I never thought DA18-55 is IF. Maybe I just didn't shoot with it for too long...

[stevebrot]

I never thought DA18-55 is IF. Maybe I just didn't shoot with it for too long...

I guess technically, it is not IF since the front element does extend. The back element does not move, however.

Edit: I changed the title and text of the original post to reflect Axl's comment. I originally asserted that the 18-55 is IF...End edit

What I am wondering is if:

1. The larger image circle of the FF lenses is biasing the meter due to excess light in the mirror box
2. That moving the rear element further into the lens barrel when close focusing acts sort of like a hood in reverse resulting in the meter shift

I know that the shift is not much and can be managed nicely in PP, but I do find it a bit disturbing none-the-less. If the meter reading shifts with focus, what reading is correct?

Does anyone care to confirm this behavior?

Steve

[jeffkrol]

Ok...

Steve

What metering mode? What are you focusing on? Scattering the light by defocusing could cause a change in intensity in some of the metering "cells".

[stevebrot]

What metering mode? What are you focusing on? Scattering the light by defocusing could cause a change in intensity in some of the metering "cells".

• Center-weighted
• AV Mode
• Manual focus
• Subject is evenly lit blank wall (satin-finish white in my spare room)
• On tripod with eyepiece blind

I noticed this while doing some controlled-environment exposure series comparing open-aperture vs. stop-down metering for various apertures with the Katzeye screen and various lenses. I had to rerun the series for one lens and was surprised to see quite different values with no apparent change in the setup. Turns out that the variable was where the focus was set.

Steve

P.S. I also did a quick test outside using the overcast sky...same result. The neighbors were probably wondering what was up with their crazy neighbor taking pictures of a blank sky...