Exposure Compensation vs. Gain

[Steve Bobilin]

Now that I have the Sigma 18-35 F1.8, I've been using the EV trick Chris Harding shared. Works great. But I don't understand exactly what it is doing. I searched about Exposure Compensation, but I'm unclear about how it works. Since I have a dumb adapter it is not affecting the lens aperture in any way. It does not seem to affect the graininess of the image, so how is it different from ISO/Gain?

BTW there is considerable Vignetting with the Sigma, particularly when using ND filters.

[Michael Liebergot]

Steve the EV effects the Gain/ISO as it either increases or decreases your Gain/ISO exposure by up to 2 stops. So if you push Gain/ISO and put it in Auto then select EV, then you can increase or decrease the exposure that the camera says is optimal.

So for example if your camera selects 6db for gain and you want to increase it you can simply select EV and then you can increase your gain in incremental steps. You would do this instead of using only the 3 selectable gain/ISO presets. However I did notice that depending on the situation the camera can't decide which Gain to select. As last night I was using my Sigma 18-35 Alpha mount with the LE-EA4 adapter (smart adapter) and my exposure kept fluctuating, so the exposure was constantly changing. This was at dusk and I guess the camera couldn't decide which exposure was correct. So I turned it to a preset and all was fine. BTW I loved using the Sigma 18-35 last night at a barn reception. I rarely had to us my light. But of course focusing in low light at 1.8 was a bear.

[Steve Bobilin]

Originally Posted by Michael Liebergot

Steve the EV effects the Gain/ISO as it either increases or decreases your Gain/ISO exposure by up to 2 stops..

I'm using a dumb adapter and using EV seems to affect shutter speed—and besides—gain is not measured in "stops," right? So I think EV must be a mathematical formula or something based on a assumed f-stop and shutter speed combination?
Because I'm using a dumb adapter, my results may be different than if the camera can communicate with the lens.

[Chris Harding]

Hi Steve

Firstly are you sure your Sigma is vignetting ?? If the lens hood is not placed correctly you will get a shadow in the corner of the frame .. I did that one evening at a wedding but otherwise as long as the hood is locked correctly it should never vignette EVER ...It's only 18mm at widest!! I'm pretty sure that your hood is causing the problem but if it still does it if you take the lens hood off, then let us know??? Seriously apart from my incorrect hood placement mine has never vignetted!!

OK, all EV does is trick the camera into thinking that the bright scene you are pointing the lens at is not really that bright or if you are shooting a fairly well lit person against the night sky you can adjust the EV so the camera doesn't think "Wow the background is so dark I need to increase the gain" Typically you would use an EV of up to +2 where the subject is in deep shadow and the camera sees bright sky behind (a backlit situation) ...that way you can fool the built in exposure meter so the subject is correctly exposed even though the background is totally washed out. The camera is using gain in these situations and adjusting itself for correct exposure ... you provide the plus or minus adjustment using the EV value. What I do is lock my cameras at 21db and run in full auto and use EV is the camera doesn't get it right!!

Chris

[Steve Bobilin]

Hi Chris
You are right about the lens hood. I removed it and yet I still yet vignetting. In the bright Hawaiian sunshine I put on two ND filters and that is when I get vignetting! We shot in full sun all morning and into the PM. No lens hood at all. (because we noticed that problem) The footage was not spoiled, but it was noticeable. The Sigma lens is a Canon EF mount with a Fotodiox Pro lens mount adapter.
You are using Nikon, right? This adapter has an "aperture," but when it is closed AT ALL I get vignetting. Your Nikon lens has an aperture ring, right? maybe that is the difference.

I found this about EV:
Exposure value - Wikipedia, the free encyclopedia
It seems to affect the shutter speed, not the gain.

Thanks so much, Chris, I would not have known that you could limit the gain. I have mine limited now to 21dB.

[Chris Harding]

Hi Steve

Nope, my Nikon lenses are DX so they have no aperture ring ..both my adapters have adjusters on them. I have never had any vignetting on the Sigma and I use it a LOT at 18mm. If you take the ND's off does the vignetting disappear? If that's the case then your only option is to use a step up ring maybe and bigger ND's?? I don't use ND's as they cause issues with my footage and I am getting tired of buying them and end up with IR contamination .... I expect really good ones (like $400 each) would be OK but OUCH!!

EV affects exposure in total so provided you don't change the manual aperture it will either drop shutter or add gain or do both ...a test with my camera inside showed 0db and 1/150 at F3.5 and if I add 1 stop of EV the shutter drops to 1/100 ..if I then add 2 stops the shutter stays at 1/100 but adds 3db gain .. I control my shutter by stopping down the lens which is probably not the best thing to do if it's really bright and I need to go past F16 as the image might degrade ...I usually just run a higher shutter most times. The way I use EV is to set my zebras at 90% and in a dark back, bright subject situation I apply negative EV until zebras are gone from the subject. In backlit situations I'll add positive EV until the background is totally blown out and add until even the subject has zebras..then back off until the subject is clear ..that way in either case I'm exposing for the subject not the background.

Maybe the answer would be to use 4"x4" ND's with a mini matte box up front??

Can you actually see the shadow from the ND filter on the image?? I'm wondering if you are maybe stacking ND's so the lens sees the top most one?? A variable ND would solve that problem

Chris