Recently Purchased Nikon AF-S 14-24mm f2.8 G ED and Compared with ...

<p>Hi Guys,<br>

<br />Just purchased Nikkor AF-S 14-24mm f2.8G ED lens, the lens is awesome, I also <br>

own Nikkor 24mm AF-D prime lens. I compared both the lenses at 24mm f2.8 on<br>

my D800, at 2.8 Aperture the D800 on 14-24 adjusted the Shutter to 1/30 but on <br>

24mm 2.4 it adjusted to 1/40 (image was a bit darker) so I went full M on 24mm<br>

and adjusted the S to 1/30 to match the image taken by 14-24mm lens while <br>

ISO-200 and WB were the same, the image from 14-24mm was much brighter and<br>

sharper than my prime lens, is this expected? I thought both the image should be the<br>

same with similar settings ....</p>

<p>The reason the 14-24mm is sharper is because it's simply a better and more advanced lens.</p>

<p>The difference in brightness can come from the mechanical aperture lever on the lens is differently adjusted.</p>

<p>Another reason would be that the 24 AF-D lens has more vignetting wide open than the 14-24. Vignetting means that all the corners are darker, only the center is bright.<br /> <br />A third reason is that the f2.8 is just a rough number and not exact. The real value is of the "brightness" of the lens is called transmission. It is quite possible that these lenses have different transmission. The difference you are seeing is 1/3rd of stop which is well within what's normal.</p>

<p>There is actually a forth reason as well and it has to do that the camera can increase the ISO (without telling you) for certain lenses as certain apertures. It's because the sensor isn't as sensitive when the light enters from an angle so it increases the ISO to compensate. I'm not sure if this happens at f2.8 at 24mm. Usually it's at larger apertures. Nobody but Nikon knows for sure.</p>

<p>Thanks for your feedback ...</p>

<p> </p>

<p>Just to clear a couple of things up.<br>

In no particular order.</p>

<p>F/2.8 is not a rough number. The simplified formula for figuring an f/stop is<br>

f-stop equals focal-length / iris-diameter<br>

For example, a 30mm iris in a 240mm lens would give an f-stop of 8.<br>

T stops can only be figured for one lens at a time. Each lens needs to be tested for its true transmission of light.<br>

The 24mm AF-D lens being a much less complex design should have less loss of light then the 14-24. A percentage of light is lost at every air glass boundary inside a lens. Lens coatings help with this light loss. Just looking at the two lens designs will give you an idea of which one will have a higher T stop.</p>

<p>Pete<br>

Do you have any proof of the camera changing the ISO on its own with particular lenses?</p>

<p>Those two are very different lenses. Nikon has not changed the optical formula on their 24mm/f2.8 since the AI version, which I used to own. That version was introduced way back in 1977. Needless to say, that optical formula is quite dated by now.</p>

<p>The 14-24mm/f2.8 AF-S is a modern complex super wide lens, but it is not even a 2x zoom. Its extreme wide end (near 14mm) is poorer. So you are comparing the strong end of a modern zoom that is close to $2000 in price against a very old fixed lens.</p>

<p>The 24mm prime has worse vignetting than the 14-24mm zoom-Nikkor at f/2.8, resulting in a different exposure and an apparently darker image. By f/5.6 the vignetting has evened out and both lenses have quite similar corner brightness, but of course the camera normally meters at open aperture and therefore gives a different exposure between the two lenses.</p>

<p>The corner definition of the 24mm prime is also a lot worse than the zoom, and never catches up on stopping down. See below for a comparison of the corner shading and definition between these two lenses. The comparison uses the Ai-S version of 24mm f/2.8 Nikkor prime, but as Shun says, the AF-D version is optically the same.</p><div></div>

<blockquote>

<p>F/2.8 is not a rough number. The simplified formula for figuring an f/stop is<br /> f-stop equals focal-length / iris-diameter<br /> For example, a 30mm iris in a 240mm lens would give an f-stop of 8.</p>

</blockquote>

<p>Maybe I should have used another word than "rough". Let's call the aperture a <strong>nominal</strong> number instead. It may not be exactly f/2.8 just as a <strong>50mm lens may turn out to be a 52mm</strong> focal length if you would measure it.</p>

<p>Look at the picture below from a Nikon repair manual that shows tolerances for the aperture opening on a 50mm lens. Without going into the actual calculation the tolerances for this lens shows that in this case<strong> f/2.8 can actually be between f/2.6 and f/3</strong>. That may not sound like much but converted to stops the difference is 0.45 EV or put another way<strong> a half stop</strong>.</p>

<p> </p>

<blockquote>

<p>The 24mm AF-D lens being a much less complex design should have less loss of light then the 14-24. A percentage of light is lost at every air glass boundary inside a lens. Lens coatings help with this light loss. Just looking at the two lens designs will give you an idea of which one will have a higher T stop.</p>

</blockquote>

<p>In theory yes. But let's not forget that the 14-24 is a zoom and as such it is in reality neither constant aperture nor constant transmission. Dxomark measured the T-stop on this lens mounted on a D800 to between T/2.9 and T/3.2 over the zoom range. Converted to stops that difference is 0.3 EV or put another way <strong>1/3 of a stop</strong>.</p>

<blockquote>

<p>Pete<br /> Do you have any proof of the camera changing the ISO on its own with particular lenses?</p>

</blockquote>

<p>Well, I found out that something was fishy about five years ago when my 50/1.2 AIS was less bright wide open than the 50/1.4G wide open. The 1.2 lens should have been a 1/2 stop brighter at least in the center. After some more test my only conclusion at the time was that the camera must do something differently internally when it knows what lens is mounted.</p>

<p>Then a couple of years ago I read the two articles below that shed some more light on what was really going on:<br /> http://www.luminous-landscape.com/essays/an_open_letter_to_the_major_camera_manufacturers.shtml<br /> http://www.dxomark.com/en/Reviews/F-stop-blues</p><div></div>

<p>ISO number is the same, I checked on the image, the big difference of course is the price tag, $400 Vs 2000, so of course there will be an image difference, but I was not expecting that much, I will make the files smaller and post them, D800 creates huge files.</p>

<p>Also thank you for very valuable info, specially about the T-Stop....</p>

<p>Interesting reading Pete.<br>

But I have not noticed the problem that you talk about with my 50mm f/1.2 AI-S or my 55mm f/1.2. I tried them this morning against my 50mm f/1.4 D using my D4<br>

I am wondering if the advancements in technology have moved beyond this problem</p>

<p>Michael, it's very interesting that you did that test.</p>

<p>Given that the D4 has large photosites I'm guessing that it should probably be amongst the better sensors for this.</p>

<p>I don't have a D4 here but I have a D600 and the 50mm f/1.4 AI, 50mm f/1.4 AF-S and 50mm f/1.2 AiS.</p>

<p>I made a few tests just now and analyzed the results with <a href="http://www.rawdigger.com/">rawdigger</a>. Looking at the raw values before they have been processed into an image is a lot more precise than looking at a jpeg or tiff. The raw values are also linear so it's easy to figure out exposure differences.</p>

<p>I came to a few conclusions.</p>

<ul>

<li>The first one is that we still have the camera's ISO increase unknown to the photographer. My guess is that newer tech is better but higher megapixels makes it worse. 24 Megapixel DX sensors are probably the worst offenders. </li>

<li>It seems to be less on full frame than on crop cameras but probably individual for each sensor. </li>

<li>I estimate around 0.4 stop @ 1.4 (50mm f/1.4 G) on the D600. Can't determine exactly without being able to lock the aperture fully open.</li>

<li>It probably depends on the lens as well, not just aperture and focal length.</li>

<li>If you tell the camera what manual lens you have on, it will also increase the ISO but not as much.</li>

</ul>

<p> </p>

<p>Pete<br>

I did a quick test last night with my 50 f/1.2 my 50 f/1.8 AFS and 55 f/1.2.<br>

Just looking at them in the RAW using rawdigger visually there was no difference between the three lenses. Looking at the RGB data given the newer 50 f/1.2 was the brightest followed closely by the 50 f/1.8AFS. The 55 was only 50 points darker then the 1.8.<br>

All the lenses where shot at f/4 with the exposure set by my L-508C</p>

<p>I need to spend a little time playing around with the program and see if I can nail down some hard numbers with it.</p>

<p>Michael,<br /> Judging from dxo's data at f/4 there should be no ISO increase. Only a tiny bit at f/2.8 and then it increases as you go to larger apertures.</p>

<p>Since you have the 50mm f/1.4 D lens a good test would be to test it wide open. First set the camera to NOT control the aperture (aperture controlled by aperture ring on then lens). Take a few shots wide open of something white, obviously in manual mode. Then rotate the lens a bit so the camera doesn't connect electrically to the lens contacts anymore. Make sure you have no lens selected from the list of manual lenses (to make sure the camera doesn't know what type of lens you have mounted). Then shoot another few shot wide open.</p>

<p>That way you have two shots with the same lens, both at f/1.4. The first few shots the camera knows about your lens and the others the camera knows nothing.</p>

<p>If the raw values are still the same, Nikon have solved the problem with light rays coming from an angle on the D4 sensor. And if they haven't the raw values will tell us how many percent or fractions of a stop the difference is.</p>

<p>PS. Using the program you can make a small selection in the center (holding shift down if I remember correctly). Then the software will give you averages for that square. I simplified a bit and looked at the green channel only as it contains most of the luminance information.</p>

<p> </p>

<p>Pete</p>

<p>Using only the 50mm f/1.4D and tricking the camera like you suggest I get a differences of 348. The image where the camera knows what it is being the ever so slightly higher value. Visually there is almost no difference. I do wonder if the lens being ever so slightly out of position if the iris is not open all of the way giving the slight advantage to the fully mounted lens.</p>

<p>Michael,<br /> The iris is always open all the way when you use the aperture ring and set it to f/1.4. Doesn't matter what you do with the lens, even take it out, it's still open all the way.</p>

<p>So I'm sure this change comes from the camera increasing the sensitivity when it know what lens you have. If it's barely visible it does sound like the D4 is extremely good in this regard.</p>

<p>What was the absolute values (so we can get a percentage increase)?</p>

<p> </p>

<p>Pete<br>

Sorry for the delay.<br>

50mm f1.4D connected<br>

R 193.4<br>

G 253.2<br>

B 120.8<br>

G2 254.5</p>

<p>50mm f/1.4D disconnected<br>

R 160.0<br>

G 212.5<br>

B 110.8<br>

G2 214.2</p>

<p>That's great Michael!</p>

<p>If we do the math we can see that we get about 20% higher raw values when the camera knows about the lens.<br>

(For example the green channel is 253.2 when "connected" but only 212.5 when "disconnected. 253.2/212.5=1.1915 => 19.15% increase)</p>

<p>If we convert that to stops we get 0.25 EV or 1/4th of a stop ( ln(1.1915)/ln(2)=0.2527 )</p>

<p>Since we are shooting at the same aperture, shutter speed and ISO setting this 0.25 EV or 1/4th stop of increase in exposure comes from the camera boosting the ISO on it's own.</p>

<p>Looking at dxomark's data on different cameras. http://www.dxomark.com/itext/insights_tstop/Tstop1.jpg<br>

We see that the D4 at 0.25 EV is actually in the top spot in this regard, better than D3S and previous cameras. Newer high megapixels cameras like the Canon 7D in the chart, doesn't seem to fare that well. I expect the 24 megapixel crop cameras perhaps do even worse as well as the D800.</p>

<p>In summary this ISO boost is perhaps most of interest when wanting to buy large aperture primes to shoot in low light. For instance when we buy a f/1.4 prime instead of a much cheaper f/1.8, we expect to get 1/2 stop more light onto the sensor. In reality we get only a fraction of that and the rest is just an increase of ISO - something we could have done on our own.</p>

<p> </p>

<p>Cruelly true:</p>

<blockquote>

<p>In summary this ISO boost is perhaps most of interest when wanting to buy large aperture primes to shoot in low light. For instance when we buy a f/1.4 prime instead of a much cheaper f/1.8, we expect to get 1/2 stop more light onto the sensor. In reality we get only a fraction of that and the rest is just an increase of ISO - something we could have done on our own.</p>

</blockquote>

<p>But then, what would happen it I went and bought one of those f/1.8 lenses. Would I get just the same behaviour as described in previous posts happening around f/1.4, but now around f/1.8 instead?<br>

F100 had only 2 aperture steps of size 1/3 to go from f/1.4 to f/2.0 on a G-lens. Now this is solved. An electrical film camera - no way to boost ISO.</p>