Trying to unravel the gain/iris/iso mystery
 

Hi Guys

The EA-50 seems to exhibit very weird combinations both indoors and outdoors to me. Firstly a little theory (from Alister Chapman not my ideas) ..OK the EA-50 has a base sensitivity of 160 ISO so that's purely a measurement of sensitivity so we can also say that's equivalent to 0db gain.

Now if we adjust the iris by one stop open the sensitivity will double so we are looking at an ISO now of 320 or an added gain of +6db ...we should also have much the same if we leave the aperture where it is and either double the ISO or raise the gain 6db right???

Using the EA-50 outdoors, the camera decided in full auto to set the aperture at F16 (with a fixed shutter of 1/50th) but also added something like 9db gain which in my maths equals an extra 1.5 stops???

Indoors I had much the same story.. the iris set itself to F4.0 but with 15db gain and when I zoomed into the group of people it stuck on the 15db gain but closed the aperture to F5.6 as it figured there was too much light?

Surely the electronics would first of all open the iris right up to F3.5 on the stock lens and then if more light was needed it would then start applying gain...I was under the impression that gain was only added after the iris "runs out"

Anyone got any views on this???? Noa who shoots in manual almost all the time might be able to answer with his "modes operandi" ...Do you set shutter first, then aperture and only then gain if required??
Unless you are shooting very low light surely then setting presets to 0db, 3db and 6db would allow you to expose 1/2 stop more or a full stop more by just using the gain switches ..if you needed any more light then surely you would simply open the lens one stop and take gain back to zero??? Or have I got it wrong?

I have also noticed that in auto, the camera shutter never sets itself any higher than 1/125?? So on a bright day here you are still hitting F16 apertures ... Manually, of course you can crank it easily to 1/250th and work with a "closer to the sweet spot" aperture.

Comments and theories guys on why the camera adds biggish gains in good light rather than adjusting iris??

Chris

Re: Trying to unravel the gain/iris/iso mystery
 

Hi Guys

A huge "DOH" from me now!! I was applying all the theory to a prime lens which will, of course work exactly as above ...I completely forgot about the stock lens being a zoom so although you have F3.5 at 18mm ....go to about half way and the maximum the lens can give you is F5.0 and then a mere F6.8 when you get to the top end ...I just wasn't thinking at all..not really like me at all. If you take a regular scene and the stock lens on auto decides that a nice exposure is F4.0 at 1/50th with zero gain...now zoom in 50% of the range and you have lost about a stop and a half of aperture so the camera can no longer get in the amount of light that it needs as the max the iris can open is maybe F5.6 so it has to add 9db of gain to expose correctly even though the outdoor scene is bright shade. I couldn't figure it out while I was shooting cutaways for speeches and wide shots needed less gain than zooms.

Now, going back to Noa's complaint about only being able to set 3 gain values in manual...surely that would not be an issue in really bright light?? It's highly unlikey, except with a very slow lens or huge zoom that you will run out of aperture but I can see the problem when the camera does need more light than the lens can supply and that indeed could be quite a high gain or iso setting to expose correctly.

I rather like the fact that you can run in manual and still have shutter, gain and iris in either manual or auto. That gives a whole lot of latitude and also gives one the opportunity to switch say iso and iris back to auto for a second to see what the camera figures is a good exposure.

I think in super bright scenes if you don't have an ND then the other alternative is to push the shutter high..in our very bright road today (it's sunny and 34 degrees C!!) the auto configures 1/100 shutter and then selects an F16 iris ....you get a much nicer picture if you push the shutter to say 1/600th which drops the aperture into a sweeter area around F5.0 ....That might cause issues with lots of motion so some but for me weddings move very slowly so a high shutter causes no ill effects.

Chris

Re: Trying to unravel the gain/iris/iso mystery
 

The way I set my f-stop and gain (or iso) with a large sensor camera is quite different then with a small sensor one, like my canon xh-a1. With the Canon I just had 3 usable gain settings, 0, 3 and 6db gain, teh shutter was always at 1/50th and when things got really dark I used 1/25th shutter.
So basically I turned the iris ring to adjust my exposure and used the gain switch only if my iris was wide open and the image was still too dark. The ND filter would then only be used in very bright shots if my f-stop was too high and when I needed to bring it down to assure I didn't get any diffraction (soft image).

With a large sensor camera and especially fast lenzes you need to think differently when setting exposure, this because the dof on your image at f1.4 or at f22 will change dramatically. I always choose the f-stop first based on the amount of dof I want to achieve and then I set the ISO to set my exposure to the lowest possible value for a clean image and I use a variable ND filter to finetune my exposure. This example is for outdoor shots only if there is sufficient light.

For indoor shots the ND filter can't be used as you loose too much light so then it's the f-stop first, then set the ISO for the right exposure and it can be that I need to change the f-stop again to get a better ISO match for the right exposure, since you have so many usable iso stops that's a real hassle assigning them to the switch each time. For dark reception that's less of an issue as you need to be at 1600 iso all the time usually but when you shoot in a mixture of different light intensites it could be that one room you need 640 iso, the other 800 and the other 1600 or anything inbetween, often you tend to go for the lowest iso possible to prevent noise from appearing to much.

Re: Trying to unravel the gain/iris/iso mystery
 

Thanks Noa

I was shooting some private stuff today and that's exactly how I was doing it ...it's a little more tricky than small sensor cameras because of the DOF ... Just for interest this site is extremely useful for weddings when you know roughly how far the main camera usually is from the couple

Online Depth of Field Calculator

I can see with a 50mm F1.8 prime that at 5 metres away from people you have just over 1/2 metre in focus but get to around 2m and you have little more than 100cm to play with which needs careful consideration if you are doing cutaways of table groups ...with the 50mm I will need to make sure my shots are a little tighter !!

Chris

Re: Trying to unravel the gain/iris/iso mystery
 

I still do much the same thing on my NX5U for the theatre stuff since the lens ramps ( F1.6 to F3.4 on the NX5U) and if one is in manual exposure the image will darken as one zooms in. So I set the iris to F3.4 and adjust with the gain or keeping iris between f3.4 and my limit of f5.6 to keep a sharp image. As I mentioned before a useful mode is set the iris in manual, gain in auto but AE shift negative to taste and spotlight setting switched on. This will limit highlights and give slight underexposure that for me in Edius is easy to correct later without loosing the highlight detail. Should work the same way with the EA50 and you can manage the DOF easier. I use this approach so that I can get the DOF to cover the whole stage so that I do not have to continually focus while zooming on stage characters. Fully zoomed in of course I need to make sure that point is in focus and have peaking on to check all the time, also set to a button.

Ron Evans .