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Promit Roy

Low light performance at deep DOF

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This is something I haven't seen a lot of people talk about, especially in a video/film context. What happens to low light performance when you need DOF that isn't quite so shallow? I'm thinking GH4 vs A7S here but feel free to chime in with whatever experiences you've had.

 

Take the A7S at f/2.8 and ISO 1600. Wonderfully clean image, substantially cleaner than a GH4 at f/2.8 and 1600. But that GH4 is delivering way more DOF thanks to the small chip. So if we equivalence up the shots a bit, our A7S is now at f/5.6 and ISO 6400. Can it still compete with the GH4 on image quality when we're applying a two stop penalty to compensate for depth of field requirements?

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So if we equivalence up the shots a bit, our A7S is now at f/5.6 and ISO 6400. Can it still compete with the GH4 on image quality when we're applying a two stop penalty to compensate for depth of field requirements?

 

Look here: http://www.dpreview.com/previews/panasonic-dmc-gh4-sony-alpha-7s/7

 

The A7s at 6400 ISO has similar noise performance to the GH4 at 1600 ISO. Perhaps slightly cleaner.

 

Also, the GH4 does not inherently have more DOF. If you put a 50mm on an A7s and then on a GH4, and shot at the same f-stop, the depth of field would actually be identical. The GH4 shot would be much more telephoto because of the smaller sensor. To get the same field of view as the A7s' 50mm on the GH4, you need to use a 25mm, which has less depth of field.

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GH4 has more (deeper) DOF for the same subject composition because you have to either:

- Be further away from the subject to get the framing (longer focal distance/closer to infinity = deeper DOF)

or

-Use a wider/shorter lens, to maintain the same subject framing at the same distance. Wider lenses have a deeper DOF.

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That's why I'm in general not so ga-ga over the 35mm shallow DoF look. Plenty of work out there where the client wants to have enough of the image sharp that the FF advantage just goes away, leaving me with the disadvantage of a unnecessarily large kit.

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Guest Ebrahim Saadawi

A smaller sensor will give you deeper depth if field, all other things being equal. But current technology for some reason is making fullframe sensors much less noisier at higher gain than m43s, therefore the m43s advantage is compemsated by stopping down the lens and increasing gain, with something like an a7s, 1d, d4s, d750 you will get nearly as clean of an image with deep dof.

So remember you can always stop down fullframe to get m43s dof but you can't open up m43s beyond the maximum aperture to get the fullframe dof.

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a smaller sensor + wider lens will usually outdo a bigger sensor + longer lens if deep dof and low light performance is important.  

 

 

However, the A7S in full frame mode, ramped up to 6400 to accommodate an f5.6 aperture on a 35mm lens will likely come close to outdoing what the gh4 will need to be set up as in 4k mode to get the same d0f/fov ratio:- 15mm @ f2.8 running at 1600iso.   there are very few 15mm lenses that will match a basic 35mm at f5.6 in resolution terms - I very much doubt there are many that will also deliver 4k into such a small area when in 4k mode. 

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But current technology for some reason is making fullframe sensors much less noisier at higher gain than m43s


More sensor area means more space available for light to hit.

 

 

there are very few 15mm lenses that will match a basic 35mm at f5.6 in resolution terms - I very much doubt there are many that will also deliver 4k into such a small area when in 4k mode. 

 

Not to mention that the performance of a lens at f/5.6 is generally better than a lens that is wide open or close to it - for example at f/2.8

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Guest Ebrahim Saadawi

Well I generally believe the reason is not more light hitting because of the larger area, because the extra light hitting the full frame edges outside the m43s crop is used to form an entire different portion of the image. So per square area they're getting the same amount of light. If you shoot a full frame still and crop to m43s area (now it's getting the same light amount as a m43s and the rest is thrown away) you get the same lowlight performance, perhaps just a tiny bit worse because the grain is magnified whilst cropping. I believe it's about the actual design of the sensor and the actual pixel ability to collect light and of course the processing, I have no idea why they just don't make crop sensor versions of their fullframe sensors and we'd get the same lowlight performance (yet lower resolution of course). Perhaps part of the reason is that they want the smaller sensors to have the exact same resolution as the larger fullframe ones for some reason, so they have to scale down the individual pixel size, or perhaps it's marketing.

If Sony is delivering that phenomenal lowlight performance from the s35 and s16 crop of their A7s then nobody has an excuse.

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Sony is delivering it because the A7s has comparatively gigantic pixels that are much more sensitive light.

 

The hard part for an SLR/video hybrid is a 12MP sensor is bad news for marketing a photo camera. 

 

Even if the IQ is great, 24MP or 36MP is simply a lot more resolute than 12MP. And at the end of the day, when someone walks into a camera shop and is presented with an A7s and A7r, and is then told one is 36MP and is $500 cheaper than the 12MP one...

There are phones that have MP counts higher than 12MP. Of course, all/most here, and most professional photographers know that pure megapixel count does not have a lot to do with the quality of the camera or image, but most enthusiast photographers don't know that, and it's not a thing you can market all that well. In addition, there are many photographers who need higher MP counts.

You either bring out a range of cameras aimed at different markets, as Sony has done, or you try and make one camera that's marketable and usable as something approaching a true hybrid. If you do the latter, you have to compromise something to be able to do it in a small body at the appropriate price.

 

A7s is by far a camera aimed at the video market, where high megapixel counts do not matter.

 

It seems to me that the A7s and GH4 are the first SLR/ILC type camera that are more geared towards (and marketed towards) being a video camera that takes good photos, rather than a photo camera that also happens to shoot decent video.

 

That market will always be more niche than the photography market that demands higher megapixel counts.

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Guest Ebrahim Saadawi

Yes that's probably the reason why smaller sensors have worse lowlight performance, because they need to maintain the same high pixel count of fullframe cameras. If the 5D mk III at 22mp they can make an APS-C rebel with the same lowlight performance at 12-14mp, just use a smaller crop of the same sensor design. I would much prefer a performance of a s35 crop from the 5D at 12ish mp rather than the rebel performance at 18 stills. Much better lowlight performance, DR and colours. It would also make it easier to get a 1:1 sampling from a 16:9 window,

the A7s in on the right track, I just need to see a scientific colour conparison between it and any other well known reference like a GH4 or 5D or Cx00. I can't believe that every footage I've seen looks bad in colour, it's either the camera's fault or the user's. Hope it's the later as it's practically perfect!

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Okay, some clarifications are in order.

 

First of all, why do people feel the need to come in and condescendingly explain the push-pull between DOF, aperture, focal length, and format? Yes, we all know that the actual reason for the increased DOF is the change to a wider focal length to create the same perspective. Thank you, captain freaking obvious and his sidekick, boy repetition. Let's move on to things people actually care about.

 

Obviously for stills, we can simply go to DXO and run the numbers. The A7S gets about two and a half stops over a GH4 according to their rankings, so after the penalty the A7S still wins. Woohoo! But for video modes, we've got different scaling, NR, and crops to deal with - and as a few people said, it's very true that the exact lens choices matter. The m4/3 lenses are largely designed to be sharp wide open though, whereas the larger format lenses often aren't. And then the GH4's output is different at 1080 and 4k, versus the A7S internal scaling, etc.

 

I guess the point is, I don't want to discuss the abstract theory behind sensor sizes, total light, quantum efficiency, all of that garbage that people waste their time arguing about. I want to hear actual in the field experiences when shooting scenes that require deep DOF with different format sizes. Might be asking for too much here, though, as that requires actually using the cameras...

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Guest Ebrahim Saadawi

Okay, some clarifications are in order.

First of all, why do people feel the need to come in and condescendingly explain the push-pull between DOF, aperture, focal length, and format? Yes, we all know that the actual reason for the increased DOF is the change to a wider focal length to create the same perspective. Thank you, captain freaking obvious and his sidekick, boy repetition. Let's move on to things people actually care about.

Obviously for stills, we can simply go to DXO and run the numbers. I want to hear actual in the field experiences when shooting scenes that require deep DOF with different format sizes. Might be asking for too much here, though, as that requires actually using the cameras...


I'll correct and try to be helpful anyway, even for anyone reading this other than you.

A wider focal length is not the only reason you get a deeper depth of field.

Depth of field is directly affected by three elements.

-Focal length if the lens
-The opeining of the lens aperture/iris
-The distance between the lens and the subject in focus.

When you use a smaller sensor you indirectly affect depth of field because you either choose a 1- wider focal length or 2- increase the distance between you and the subject, to get a similar field of view, in both cases the amount of area in-focus is decreased.

We're not giving you an abstract answer, we are saying it depends on the cameras you're comparing. Let me give you an exanple to make it clearer:

A 5D mk I original vs a GH4. To get the same deep depth of field as you'd do on the GH4 in lowlight you will have to shut the iris, pump up the gain and you will get a noisier image on the 5D but with same dof.

An A7s vs GH4, to get the same deep depth of field as you'd do on the GH4 you will stop down the lens and pump up the gain, you will get the same deep dof along with a cleaner image.

It totally depends on the sensor technology. But generally if you're comparing two cameras from the same generation the full frame cleaner image at high ISO compensates for the iris closing you'll have to do to get a m43s/s35 image. So at the same generation there isn't usually an advantage to the smaller sensor.

Tell us what exact two cameras you have in and we'll see how they compare in getting a deep depth of field in lowlight situations.

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Guest Ebrahim Saadawi

By the way I am personally offended by your last comment directed at all of us.

especially the parts saying:

"That I am condinscendingly speaking,
captain freaking obvious and his sidekick,
Boy repetition,
Me talking about things nobody cares about,
and wasting my time arguing garbage
and that we don't have experience shooting video and using cameras"

And I would only put this behind me with an apology and explanation why you'd say such things whilst we're just trying to help you for absolutely no return. Otherwise I am not commenting on any word written by you here or anywhere else again.

Good day, Promit.

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lt's keep this nice and friendly guys! 

 

in answer to your innital question -

 

this is one of the reasons I don't like full frame as you do end up having to stop down to f4 - f5.6 range in general to make it match the dof of apsc / 4 perf 35mm film cameras etc .

I prefer to push a lens into its sweet spot of artistic look ....full wide open or just one stop down from there thats where I find the magic of a lens is looks wise...not at clinically perfect f5.6 - at which all lens are pretty good.

 

So stopped down to f5.6 in a low light situation is not so good as you either add more lights to the scene/ location (if you artistically can do that without altering the look too much )

 

or you increase the iso ,

 

I dont ever like to go over 800 iso on my Panasonics , I shoot f2 in low light as thats just enough dof without it getting silly narrow .

 

Thats why my Canons are only used for stills every now and then  and I shoot everything on micro 4/3 with speedboosters.

I prefer a smaller sensor not full frame.

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Well you could use medium fromat lenses then, if you ever wanted to use FF. For example, the pentax 6x7 55 3.5 is a pretty soft lens until f11 but some say it has a nice look to it.

The only problem is that you won't find anything wider than 40mm and that these lenses aren't "compact". (all these lenses are super cheap) 

Who wants to go wider than 40mm (on 36mm wide) anyway when talking about cinematic :P

 

Before I forget, you can put aluminium paper stripes into the adapter, this will give some serious contrast/streakes alterations because the image circle is so big and gets reflected onto the sensor if you give it the chance.

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I prefer to push a lens into its sweet spot of artistic look ....full wide open or just one stop down from there thats where I find the magic of a lens is looks wise...not at clinically perfect f5.6 - at which all lens are pretty good.

Interesting aspect.

 

Especially for me as a pure run&gunner I find myself shooting pretty often at f2.0-f4.0 - although i could use f0.95 (using the Panasonic GM1 & Voigtländer 25mm).

So if i followed this topic correctly: if I shoot my lenses on m43 mostly wide open (or pretty close to that), FF like the A7s makes little sense?

 

Initially i planned to upgrade to FF eventually, but this is taking a complete different turn...

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It's not just pixel size anymore - modern sensors don't really have that limitation as much due to much better design (gapless microlenses for example). How the sensor is designed and what is it optimised for plays a part too. The 1DX, D4S and A7S all have sensors designed for much better performance at higher ISOs at the cost of base ISO performance.

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