Does FOV affect image quality?

[Daniel Cegla]

I was reading the DSMC primer on the FOV and crop factors, here:

http://reduser.net/forum/showthread.php?p=519532#post519532

And my main question when reading that was this:

Let's say I've got that 80mm on an S35 in the example. I see it is more telephoto than the FF35 (the full frame gives a wider FOV with the 80mm since it uses more of the lens - right?). Well, wouldn't it produce the same result with the S35 if I just took a couple of big steps back, and thus give me the same FOV?

What I am asking is, does the larger sensor size just allow you to use a certain lens in tighter quarters to keep things wider? Or is there an image quality advantage in terms of what you get out of a particular lens/shot when you don't have as big of a crop factor?

Just not sure if I fully understand why I would REALLY be inclined to get an FF35 over an S35 (not speaking in terms of resolution or DR, just in terms of lenses/crop factor)?

 

[Zhibo Lai]

Taking a few steps back to match the FOV of a S35 to the FF35 will not give you the same picture. Once you start stepping back you've now changed the distance the camera is to the subject and thus the subject has shrunk a little and perspective has shifted a little.

 

[David Rasberry]

When you move, your perspective changes. Wider lens on a smaller format maintains the same perspective at the same FOV and distance. Some say you get a better sense of depth and dimensionality with the longer lens on the larger format though.
The tradeoff is that the larger the format and longer the lens, the more difficult it is to focus on a moving object.

 

[Daniel Cegla]

If you use a lens that is properly sized for an S35 that is 20mm, is it the same as a lens properly sized and designed for an FF35 that is 20mm? Since it is sized right, it would remove the crop factor, and it should look identical? And likewise, if you use a mini prime that is sized for a 2/3rd Scarlet, should you maintain the same distance/FOV/perspective with a 20mm when each lens matches the sensor size? So crop factor is irrelevant?

Or am I missing something?

 

[Justin O'Neill]

Let's say I've got that 80mm on an S35 in the example. I see it is more telephoto than the FF35 (the full frame gives a wider FOV with the 80mm since it uses more of the lens - right?). Well, wouldn't it produce the same result with the S35 if I just took a couple of big steps back, and thus give me the same FOV?

If you take a few steps back you will get the same field of view but like David said, your perspective will change. Otherwise the quality of the image will remain the same.

Now if you put an FF35 or S35 lens on the 3k Scarlet they might not be as sharp when viewed @3k as the miniRED primes and other lenses designed for the 2/3" format. This is because FF35 and S35 lenses were not designed to resolve that kind of pixel density (3k horizontal pixels on 2/3"). If the 3k footage is downrezzed to 1080 I doubt you could tell the difference, however.

 

[Craig Parkes]

If you use a lens that is properly sized for an S35 that is 20mm, is it the same as a lens properly sized and designed for an FF35 that is 20mm? Since it is sized right, it would remove the crop factor, and it should look identical? And likewise, if you use a mini prime that is sized for a 2/3rd Scarlet, should you maintain the same distance/FOV/perspective with a 20mm when each lens matches the sensor size? So crop factor is irrelevant?

Or am I missing something?

You are missing something. A 20mm is a 20mm is a 20mm.

How they look on different formats is whats different. It's just the film back/sensor and the mounts changing, not the actual lenses characteristics.

Crop factor is only a comparative thing. I.e a super35mm motion picture film back/sensor will crop a 50mm lens differently to a 35mm full frame stills back/sensor.

Both back/sensors are 'cropping' the 20mm to some degree. If they weren't then you'd see vignetting or even the whole image circle.

I believe (and someone would need to clarify this for me) that lenses that use glass that provide larger coverage don't change the characteristics of the image that you see in terms of field of view, they just extend the edges out so that they won't be cropped on the larger sensors.

 

[Justin O'Neill]

If you use a lens that is properly sized for an S35 that is 20mm, is it the same as a lens properly sized and designed for an FF35 that is 20mm? Since it is sized right, it would remove the crop factor, and it should look identical?

No, a 20mm lens sized for FF35 has MORE glass around the edges so to speak and will thus have a wider field of view when used on an FF35 sensor that can "see" that glass. The depth of field remains exactly the same though.

Focal length is the distance from the sensor to the optical center of the lens. Depth of field is directly tied into focal length. Focal length and depth of field are absolutes and remain the same (given the same f-stop) whether the lens is 2/3", S35, FF35 or Medium Format.

 

[Justin O'Neill]

I believe (and someone would need to clarify this for me) that lenses that use glass that provide larger coverage don't change the characteristics of the image that you see in terms of field of view, they just extend the edges out so that they won't be cropped on the larger sensors.

If you put a 2/3" 20mm lens on a 3k Scarlet and an FF35 20mm lens on the Scarlet you are correct, the field of view would be exactly the same.

 

[Daniel Cegla]

Well, that certainly makes more sense now. I didn't realized the focal length was the distance from the glass element to the sensor. And then the actual physical size of the glass doesn't affect perspective or Field of View or how wide a shot is, it just allows it to fit a bigger sensor. Check.

So for a long time I thought DoF had to do with how large a sensor is and how much light it can let in. But from what I read now, that is NOT the case. It was partially true, but only because the sensor size was related to what kind of lens focal length you could use for a given shot. Is that correct? DoF has *nothing* to do with the sensor size, and EVERYTHING to do with the lens and the aperture than lens is at. The larger sensor size just ENABLES you to use certain lenses and certain focal lengths/apertures to create the desired DoF, FoV, perspective, etc. - is that correct? And thus the 2/3rd inch and smaller sensor sizes just limit the flexibility you have with lens choice/aperture @ a given DoF?

 

[Jacek Zakowicz]

confusion

confusion

No, a 20mm lens sized for FF35 has MORE glass around the edges so to speak and will thus have a wider field of view when used on an FF35 sensor that can "see" that glass. The depth of field remains exactly the same though.

Focal length is the distance from the sensor to the rear glass element in a lens. Depth of field is directly tied into focal length. Focal length and depth of field are absolutes and remain the same whether the lens is 2/3", S35, FF35 or Medium Format.

Justin, please,
focal lenght is not the distance from rear element to sensor and dof is not tied to focal lenght. you are confusing things unnecessarily.
both terms can be looked up on the net easily enough...
Otherwise you are correct... just as long as the term"more glass around the edges " is not taken literally..:thumbsup:

 

[Justin O'Neill]

Justin, please,
focal lenght is not the distance from rear element to sensor and dof is not tied to focal lenght. you are confusing things unnecessarily.
both terms can be looked up on the net easily enough...
Otherwise you are correct... just as long as the term"more glass around the edges " is not taken literally..:thumbsup:

I apologize. Is the term "optical center" of the lens a better description?

Discussing the effects of focal lengths on different sensor formats seems extremely hard to do. Because of all the sensor formats in use these days (1/3", 1/2", 2/3", S35/APS-C, FF35) I constantly have shooters asking me what lenses they can use and how it effects depth of field and field of view.

The problem is this is such a complex equation with many different factors effecting the final result. How do we describe this so it makes sense to people who haven't had to deal with different formats before?

I am surprised that you say depth of field is not tied to focal length. If the f-stop is the same, won't a lens with a smaller focal length always have deeper depth of field regardless of sensor size?

 

[Jacek Zakowicz]

terminology

terminology

No worries Justin, we just need some tighter terminology, otherwise we'll go on the limb quickly.
Optical center sounds much better since for retrofocal lenses it's actually behind the wide's rear element- outside of the lens.
The DOF is not really tied to FL -its tied to magnification and f stop.
for example regardless of the fl two lenses with the same f stop projecting object at the same magnification will have the same dof for all practical purposes. If you take extreme cases like a 10mm and 1000mm they will look totally different because of the perspective but the dof will be the same. of course the perception will be dramatically different since the 10mm will tremendously stretch the perspective while the 1000mm will completely flatten it but the dofs will match

 

[Daniel Cegla]

So..... basically Justin had it right, but OptiTek, you are saying if one is to be very technical about it, the DOF is the same, you just can't tell when using a wider lens?

ALSO:

Would you say the technical factors of that make up the look of an image (other than shot composition/lighting - just as it relates to the camera itself) are:

- Perspective (determined by distance), PLUS
- Field of View (determined by sensor size), PLUS
- Distortion (right term? what I mean is, widened or flattened. Determined by focal length), PLUS
- Depth of Field (determined by focal length and aperture)

?

 

[Doug Horton]

No worries Justin, we just need some tighter terminology, otherwise we'll go on the limb quickly.
Optical center sounds much better since for retrofocal lenses it's actually behind the wide's rear element- outside of the lens.
The DOF is not really tied to FL -its tied to magnification and f stop.
for example regardless of the fl two lenses with the same f stop projecting object at the same magnification will have the same dof for all practical purposes. If you take extreme cases like a 10mm and 1000mm they will look totally different because of the perspective but the dof will be the same. of course the perception will be dramatically different since the 10mm will tremendously stretch the perspective while the 1000mm will completely flatten it but the dofs will match

this terribly confuses me, since essentially, say we have a scenic landscape, with a cactus around 10 ft away, and a rocky skyline behind it. now the 10mm will give us a nice wide shot where both the cactus and the skyline in the distance are sharp (or at least the image appears so), but the 1000mm lense would be punched in on the cactus, with a few needles in focus, and the skyline would barely be visible due to the seemingly extremely shallow dof, correct?

 

[Jacek Zakowicz]

dof

dof

incorrect:
To achieve the same magnification on both lenses you'd have to move the lens away from the subject. Your example has the numbers play out very nicely:
If you photograph the above mentioned cactus from 10ft with the 10mm lens you'd have to photograph the same cactus from 1000ft away to get the same magnification with the 1000mm lens. If the stops are the same so will be the dof. The perspective will make the pics look different- the landscape on the horizon will appear much closer(to the cactus) on the image generated by the 1000mm lens- that's what I mean by flat perspective..

 

[David Rasberry]

incorrect:
To achieve the same magnification on both lenses you'd have to move the lens away from the subject. Your example has the numbers play out very nicely:
If you photograph the above mentioned cactus from 10ft with the 10mm lens you'd have to photograph the same cactus from 1000ft away to get the same magnification with the 1000mm lens. If the stops are the same so will be the dof. The perspective will make the pics look different- the landscape on the horizon will appear much closer(to the cactus) on the image generated by the 1000mm lens- that's what I mean by flat perspective..

So a larger sensor requires more magnification, hence a longer lens, to achieve the same field of view at the same distance as a smaller sensor?

 

[Justin O'Neill]

incorrect:
If you photograph the above mentioned cactus from 10ft with the 10mm lens you'd have to photograph the same cactus from 1000ft away to get the same magnification with the 1000mm lens. If the stops are the same so will be the dof.

Ok I am going to have to ruminate on this overnight and see if I can wrap my head around it.

So would you disagree that depth of field refers to the section of an image that appears acceptably sharp or in focus?

 

[Jacek Zakowicz]

COC

COC

Absolutely- acceptable focus only. Do you really want to start the circle of Confusion discussion here? As if we were not confused enough...

 

[Doug Horton]

god....

 

[Nick Gardner]

If you have ever been an AC, it makes perfect sense. Some would think it would be harder to pull focus on a CU on an 85mm lens than a 25mm lens. If the framing is the same, the dof is exactly the same, and there for, pulling focus is just as hard on the 25 as on the 85mm.

Look at it this way, you are doing a wide shot of 3 guys sitting around a campfire. You are on an 18mm, but fairly close. Now, if you frame up the same shot on a 300mm. You are now so freaking far away, that almost everything is in focus (think like you are almost at infinity on the lens barrel) around the camp fire, just like it was on the 18mm.

Nick