AF system: D7200 vs D750

Derma pro said:

Although the AF module is similar in these two cameras, it seems there is a difference in sensitivity of the AF points to light. Both have 51 points and 15 point cross type. What are the differences between them?

1. Which can pick AF more easily in low light?

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Derma pro said:

2. Which is better for tracking moving subjects both in good and low light?

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Derma pro said:

3. It is said the D7200's 51 points are all EV-3 sensitive. What about D750 in this regard?

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Derma pro said:

4. It is written that the center AF in D5100 for example can pick focus till f/5.6

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Derma pro said:

What does this mean? And what about the relation between the Aperture and focus points response in D7200 and D750?

I appreciate any help in understanding this issue.

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romfordbluenose said:

Yes Af sensitivity can be decision point.

Lens af wide open so the f8 corresponds to the minimum f stop of the lens. The lens you are using meet this criteria, in fact all lens do unless they are very old or exotic. Its when TCs are attached that some lens then do not meet the f8 criteria.

Also the maximum aperture of the lens does affect AF capability as an f2.8 lens lets in a lot more light than a f5.6 lens which then needs the af module to be more sensitive.

The second aspect is sensitivity. The D750 at -3EV can focus in a lot less light than a camera with a sensitivity of -1.

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Derma pro said:

Thanks so much for the explanation.

What I am confused of is the f number. A lens like the 18-105 has f/3.5-5.6. So the wider aperture is 3.5 without any zoom and can stop to f/22 (with full zoom it ranges from f/5.6-36). To say the AF point is till f/5.6 or f/8 ,and with D5100 and D5300 that is only for the central point, is somewhat confusing.

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Derma pro said:

Does this have any relation with telling the AF sensitivity is -3 EV for all points in D7200 (and -1 EV for central point in D5300)?

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Derma pro said:

What is the range of f/number meant to allow AF?

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Derma pro said:

Does this f/number thing restricted to central point in all cameras? Or the higher lines than the D5xxx not restricted to the central point?

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Derma pro said:

I am concerned with this as I like to have a reliable AF system. I am not a pro but I find difficulty to adjust focus in low light even with the AF illuminator On in my D5300 if the distance with subject is more than 2.5 meters approx. I have a chance for change and thinking of D7200 or D750. The other option is to keep the D5300 and get better lenses. I am asking if this issue of AF sensitivity and accuracy can be a decision-making and worth the change for now? And if having D750 is a great improvement in the AF that worth the full frame shift?

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Derma pro said:

romfordbluenose said:

Yes Af sensitivity can be decision point.

Lens af wide open so the f8 corresponds to the minimum f stop of the lens. The lens you are using meet this criteria, in fact all lens do unless they are very old or exotic. Its when TCs are attached that some lens then do not meet the f8 criteria.

Also the maximum aperture of the lens does affect AF capability as an f2.8 lens lets in a lot more light than a f5.6 lens which then needs the af module to be more sensitive.

The second aspect is sensitivity. The D750 at -3EV can focus in a lot less light than a camera with a sensitivity of -1.

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Thanks a lot for the reply.

One thing: isn't the minimum f stop of the 18-105 lens is f22? How f8 is correspond to the minimum f stop of the lens?

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Derma pro said:

In other words: concerning AF points, what are considered minimum and maximum f stops for the 18-105 lens?

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BirgerH said:

Derma pro said:

Thanks so much for the explanation.

What I am confused of is the f number. A lens like the 18-105 has f/3.5-5.6. So the wider aperture is 3.5 without any zoom and can stop to f/22 (with full zoom it ranges from f/5.6-36). To say the AF point is till f/5.6 or f/8 ,and with D5100 and D5300 that is only for the central point, is somewhat confusing.

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No. The cameras always focuses wide open (widest aperture possible) when speaking Viewfinder mode. With all the points.

BirgerH said:

Does this have any relation with telling the AF sensitivity is -3 EV for all points in D7200 (and -1 EV for central point in D5300)?

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Actually yes. The f:number limitation is just the aperture, where the light hitting the AF-sensor starts being less - it's not an absolute limit for the ability to auto focus. The sensitivity describes, when available light is too small, even with an aperture at f:5.6.

BirgerH said:

What is the range of f/number meant to allow AF?

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Again - every lenses focuses with their widest aperture - the AF- sensor specifications allows that to be wider than or equal to f:5.6. The f:8 specification was a result of the D7100 central point being one stop more sensitive.

BirgerH said:

Does this f/number thing restricted to central point in all cameras? Or the higher lines than the D5xxx not restricted to the central point?

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No. The f:5.6 refers to all consumer cameras for all their focus points. Some cameras have points (central, normally) with higher sensitivity, allowing to focus lenses with smaller max. aperture than f:5.6 - if light conditions are well - using those points. That's the case with the D7100 and D7200

BirgerH said:

I am concerned with this as I like to have a reliable AF system. I am not a pro but I find difficulty to adjust focus in low light even with the AF illuminator On in my D5300 if the distance with subject is more than 2.5 meters approx. I have a chance for change and thinking of D7200 or D750. The other option is to keep the D5300 and get better lenses. I am asking if this issue of AF sensitivity and accuracy can be a decision-making and worth the change for now? And if having D750 is a great improvement in the AF that worth the full frame shift?

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That's of course personally (the bolded) - and may be a matter of ones primary or casual needed works (or wants ).

The AF-systems of the D7200 and/or the D750 are better than the one of the D5300 - that's for sure - and this is not automatically helped by better lenses

Lenses with bigger max. apertures is good for low light situations - yes - but - It doesn't help you for focusing, though - because your focus system does not use the wider apertures - putting a f:1.8 lens on your camera makes no difference to the AF-system

You could try:

Using LiveView, set the lens wide open for focusing - and then change the aperture for a desired value before shooting. In that case, "better" lenses can be a help.

BirgerH.

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Derma pro said:

Thanks so much for this very informative reply with points I really needed to know in this clear way.

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Derma pro said:

Derma pro said:

Derma pro said:

I am concerned with this as I like to have a reliable AF system. I am not a pro but I find difficulty to adjust focus in low light even with the AF illuminator On in my D5300 if the distance with subject is more than 2.5 meters approx. I have a chance for change and thinking of D7200 or D750. The other option is to keep the D5300 and get better lenses. I am asking if this issue of AF sensitivity and accuracy can be a decision-making and worth the change for now? And if having D750 is a great improvement in the AF that worth the full frame shift?

Click to expand...

Click to expand...

Thanks so much for this very informative reply with points I really needed to know in this clear way.

Click to expand...

Derma pro said:

romfordbluenose said:

Yes Af sensitivity can be decision point.

Lens af wide open so the f8 corresponds to the minimum f stop of the lens. The lens you are using meet this criteria, in fact all lens do unless they are very old or exotic. Its when TCs are attached that some lens then do not meet the f8 criteria.

Also the maximum aperture of the lens does affect AF capability as an f2.8 lens lets in a lot more light than a f5.6 lens which then needs the af module to be more sensitive.

The second aspect is sensitivity. The D750 at -3EV can focus in a lot less light than a camera with a sensitivity of -1.

Click to expand...

Thanks a lot for the reply.

One thing: isn't the minimum f stop of the 18-105 lens is f22? How f8 is correspond to the minimum f stop of the lens?

In other words: concerning AF points, what are considered minimum and maximum f stops for the 18-105 lens?

Click to expand...

romfordbluenose said:

Derma pro said:

romfordbluenose said:

Yes Af sensitivity can be decision point.

Lens af wide open so the f8 corresponds to the minimum f stop of the lens. The lens you are using meet this criteria, in fact all lens do unless they are very old or exotic. Its when TCs are attached that some lens then do not meet the f8 criteria.

Also the maximum aperture of the lens does affect AF capability as an f2.8 lens lets in a lot more light than a f5.6 lens which then needs the af module to be more sensitive.

The second aspect is sensitivity. The D750 at -3EV can focus in a lot less light than a camera with a sensitivity of -1.

Click to expand...

Thanks a lot for the reply.

One thing: isn't the minimum f stop of the 18-105 lens is f22? How f8 is correspond to the minimum f stop of the lens?

In other words: concerning AF points, what are considered minimum and maximum f stops for the 18-105 lens?

Click to expand...

We talk about maximum aperture for a wide open lens and a minimum (smallest) f stop for a wide open lens. i.e. The minimum f stop of a f2.8 zoom lens is f2.8 which gives the maximum aperture. Sorry just terminology.

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mosswings said:

Derma pro said:

mosswings said:

mosswings said:

I am concerned with this as I like to have a reliable AF system. I am not a pro but I find difficulty to adjust focus in low light even with the AF illuminator On in my D5300 if the distance with subject is more than 2.5 meters approx. I have a chance for change and thinking of D7200 or D750. The other option is to keep the D5300 and get better lenses. I am asking if this issue of AF sensitivity and accuracy can be a decision-making and worth the change for now? And if having D750 is a great improvement in the AF that worth the full frame shift?

Click to expand...

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Thanks so much for this very informative reply with points I really needed to know in this clear way.

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Derma - Birger has given you the essential information you need WRT the max aperture rating of an AF system. It's horribly confusing to most. I still have trouble explaining it, which says that I still haven't fully groked it either. But our Marianne Oelund has, and explains it all, here:

http://www.dpreview.com/forums/post/54211961

Really all that the max aperture rating is describing is where on the lens the system is looking to obtain its AF signals; the f/5.6 number is a restatement of the angle of view, which is set to allow the system to work with all lenses provided by the manufacturer (you'll notice that all Nikon lenses never have less than a maximum aperture of f/5.6). The problem is when you put a teleconverter on one of those lenses...this slows down the lens by 1 or more stops depending on the magnification of the TC, and that means that the AF may stop working well, or not work at all, particularly in low light conditions. So most lenses that can accept teleconverters are f/4 or f/2.8, because that results in a TC+lens combo that meets the f/5.6 AF limitation. Nikon began introducing the f/8 AF points on its more recent cameras to allow stronger TCs to be used on its f/4 lenses, and on its variable aperture telezooms like the 80-400 f/4-5.6. Birger correctly points out that the central f/8 focus zone had to also increase its light sensitivity to do this.

Your observation that the AF illuminator doesn't help if you try to focus on subjects more distant than 2.5 meters is understandable...it doesn't have the range to illuminate a larger scene. While an additional stop or two of AF sensitivity will definitely help in AF capture, it has its limits as well. The D7200 will focus better than the D5300, and the D750 better than the D7200, but for different reasons. The D750 and D7200 use much the same AF module, the D5300 uses a lower-grade one.

My point is this: AF system sensitivity is a useful limiting specification, but may not directly indicate AF performance at the light levels typically encountered in what we call "low-light" situations. As Birger has noted, EV-2 and EV-3 light levels are moonlight-on subject, no city lights; EV1 is a distant lighted skyline; EV4-5 are candlelit interiors and the more typical conditions under which AF systems slow and struggle enough that we can't capture rapidly moving objects. You will see a noticeable improvement in these conditions with a better camera, but your D5300 is already pretty good in that department; the differences may not be earth-shattering.

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BirgerH said:

Derma pro said:

Thanks so much for this very informative reply with points I really needed to know in this clear way.

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Yes - forgot:

"Does this have any relation with telling the AF sensitivity is -3 EV for all points in D7200 (and -1 EV for central point in D5300)?"

It's not all the points of the D7200, that is sensitive to -3EV - it's the central point only - the rest is sensitive to, as far as I recall, -2EV. The D7100 has the central point as -2EV and the rest -1EV.

The sensitivity is related to the light passing through the micro-lenses in the AF-system - which is f:5.6 - meaning that, if you use a f:8 lens at the central point (-3EV) - you might not have enough light to focus, if light conditions are less than -2EV.

-2EV is the light from a nightscene illuminated with full moon. Even you eyes might have troubles focusing then.

As far as I know, all the points in the D5300 are -1EV, not the central only.

BirgerH.

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Derma pro said:

mosswings said:

Derma pro said:

Derma pro said:

Derma pro said:

I am concerned with this as I like to have a reliable AF system. I am not a pro but I find difficulty to adjust focus in low light even with the AF illuminator On in my D5300 if the distance with subject is more than 2.5 meters approx. I have a chance for change and thinking of D7200 or D750. The other option is to keep the D5300 and get better lenses. I am asking if this issue of AF sensitivity and accuracy can be a decision-making and worth the change for now? And if having D750 is a great improvement in the AF that worth the full frame shift?

Click to expand...

Click to expand...

Thanks so much for this very informative reply with points I really needed to know in this clear way.

Click to expand...

Derma - Birger has given you the essential information you need WRT the max aperture rating of an AF system. It's horribly confusing to most. I still have trouble explaining it, which says that I still haven't fully groked it either. But our Marianne Oelund has, and explains it all, here:

http://www.dpreview.com/forums/post/54211961

Really all that the max aperture rating is describing is where on the lens the system is looking to obtain its AF signals; the f/5.6 number is a restatement of the angle of view, which is set to allow the system to work with all lenses provided by the manufacturer (you'll notice that all Nikon lenses never have less than a maximum aperture of f/5.6). The problem is when you put a teleconverter on one of those lenses...this slows down the lens by 1 or more stops depending on the magnification of the TC, and that means that the AF may stop working well, or not work at all, particularly in low light conditions. So most lenses that can accept teleconverters are f/4 or f/2.8, because that results in a TC+lens combo that meets the f/5.6 AF limitation. Nikon began introducing the f/8 AF points on its more recent cameras to allow stronger TCs to be used on its f/4 lenses, and on its variable aperture telezooms like the 80-400 f/4-5.6. Birger correctly points out that the central f/8 focus zone had to also increase its light sensitivity to do this.

Your observation that the AF illuminator doesn't help if you try to focus on subjects more distant than 2.5 meters is understandable...it doesn't have the range to illuminate a larger scene. While an additional stop or two of AF sensitivity will definitely help in AF capture, it has its limits as well. The D7200 will focus better than the D5300, and the D750 better than the D7200, but for different reasons. The D750 and D7200 use much the same AF module, the D5300 uses a lower-grade one.

My point is this: AF system sensitivity is a useful limiting specification, but may not directly indicate AF performance at the light levels typically encountered in what we call "low-light" situations. As Birger has noted, EV-2 and EV-3 light levels are moonlight-on subject, no city lights; EV1 is a distant lighted skyline; EV4-5 are candlelit interiors and the more typical conditions under which AF systems slow and struggle enough that we can't capture rapidly moving objects. You will see a noticeable improvement in these conditions with a better camera, but your D5300 is already pretty good in that department; the differences may not be earth-shattering.

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I am very thankful for the explanation and the link. It made a great difference to have your and Birger's valuable explanations.

It appeared to be very important to have a knowledge of how the AF system is working and its sensitivity. And to know the advantages/limitations of my camera's AF system. This apparently helps to get better understanding of how to make better photos especially in low light and to think better of the actual needs when upgrading the camera body.

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