Nikon lens not what it purports to be

[Rod Lawton]

Originally Posted by DouglasMorley

Greenwing,

The EXIF data tells me, and always has told me that the lens's extremes were 18mm and 70mm. On that basis alone I have never had any reason to doubt them as I have never directly compared this lens to similar optics. I feel pretty sure that very few owners and users have either - and that might well include lens testers from publications as well.

As Stephen Batey rightly points out a 1mm difference is hardly worth anyone's concern so 17mm or 18mm probably would not get noticed. However 50mm against 70mm is somewhat more extreme. Either all 18-70 Nikkors are in fact 17-50mm and everybody's been duped, or I have a rogue.

Without input from a 18-70mm lens owner, I'll never know.

I have an 18-70mm, an 18-55 and a 55-200 but I've not yet checked them against each other. We are thinking about whether focal length measurement should be incorporated into in our lens tests but we'd want to be sure there was enough variation amongst lenses as a whole to make this a standard fixture.

As greenwing has pointed out, the tests would need to allow for variations in magnification with focus distance.

Please don't be discouraged by any lack of response in the forums. Bear in mind that forums these days face a lot of competition from social media such as Facebook, Flicker and countless other photo related sites (and other forums). The problem isn't that people don't have anything to say, but there are just so many places to say it!

For the record, I've never had a terribly high opinion of the 18-70mm. The extra zoom range is good (even if that's a moot point in this thread!) but there's no VR and my example isn't brilliant optically - I think the later 18-55mm VR is a better budget all-rounder, but I like the 16-85mm too.

RL.

[DouglasMorley]

Thank you Rod for your reply.

I agree with you about the 18-55mm kit lens. Actually, using the test card the best of the three kit lenses that I tested - 18-70, 18-55mmVR and the older 18-55mm, was the older 18-55 without the VR.

Regarding the lack of response, although I appreciate your points, I do believe that generally the various media that you mention are filled with vacuous drivel most of the time leading me to think that intelligent dialogue could well be a dying aspect of a lot of written communication. I used to think that people didn't comment on images because they were frightened of offending. Now I think that they mostly have difficulty stringing words together in one sentence let alone two or more. As for their spelling and grammar.........

Am I being cynical or are the number of people able to fully understand me diminishing rapidly?

[StephenBatey]

On the original topic, I can provide a reasonable explanation that would account for the differences between the image size at a nominal 70mm that you found with the 18-70mm and the 55-200mm lenses. Can, but won't as it involves maths that I've found most people don't want. So, although I hate saying this (because it's entirely possible that I've made a slip and if I gave my workings it could be checked) you'll have to take my word for it.

On related topics, I've found that photographers have a "herd instinct" in what matters to them. Megapixels, noise at high ISO are important to them, even if they never make large enlargements or use high ISO settings, or rarely do. How big a subject brightness range their cameras can successfully record (even though it affects every single photograph they take) is of no interest. I once asked on another forum if anyone had ever tested their camera. The only person that ever commented (a professional) said he had no idea and had never checked. So, don't expect people to have any interest in a topic just because it's of immense practical importance.

You may be right that people have difficultly stringing words together, but two points to bear in mind:

1. The internet opens you up to seeing written communications from people whose writing you would never have seen before.

2. Educational standards have dropped seriously from former times.

As to comments - well, I can string words together reasonably well, and I don't make comments on people's photos now unless explicitly asked, simply because I am too plain and easy to understand. I usually offend simply because I point out what should have been obvious, but no-one else either sees or dares to mention.

Finally - I don't know if the number of people able to fully understand you is diminishing, but again consider the nature of forum communications. It's not like receiving a letter, where you can read carefully and reply at leisure. The immediacy encourages a fast read and a fast response. and that in turn encourages people to assume that they understand without actually reading. If you use certain words, it's more likely to trigger an emotional response that will block out your true meaning, as I found on another forum. I mentioned in a thread that I'd had bad treatment while in hospital. This triggered (apparently) the following sequence of thoughts: "bad treatment = criticism; criticism = wants to dismantle the NHS" and a full blooded attack on my principles for wanting to privatise the NHS followed on my post.

Back on topic, if you want a reasoned answer to your query (so that you can see if you agree) let me know and I'll send one privately.

[DouglasMorley]

Stephen,

Thanks for your considered reply. Your comment about educational standards is certainly true to my way of thinking. If you were subjected to a bad NHS experience, I feel for you. On the health and education fronts (and several others), many politicians over many years could and should be held to serious account, but they never will be.

As to the technical and mathematical reasoning behind my original query, you are, by and large quite right in that few photographers/picture takers care as long as they get what they want from their equipment. Frankly I've no desire to get down to the scientific reasons. The mere fact that what I had assumed to be was not, took me most by surprise.

Leaves one thinking though that, although generally "the proof of the pudding is in the eating" - how are we to know that the ingredients on the side of the box are really in it?

[StephenBatey]

As a non-mathematical answer to your final question (and assuming that we're referring to lenses) the simple answer is that you can believe the packaging. But only when you use the product under the conditions to which the figures apply. Here's an illustration that most photographers should be familiar with, based on the maximum aperture.

To keep things very simple, we won't even consider zoom lenses which may have a variable (and specified) maximum aperture, and just look at prime lenses. Your lens is marked to be f/2. You take a meter reading, and see that at f/2 the exposure time should be (say) 1/1000th second. You make the exposure, and we'll assume that a) it's on film and b) you have a densitometer available. You measure the effect of 1/1000th at f/2 and find that the exposure is perfect. Your f/2 lens really is f/2.

Or is it? Refocus on a really close object, so close that it's reproduced at 1:1 on the film. The light is exactly the same, so the exposure should still be 1/1000th at f/2, shouldn't it? Take the photo, and you'll find that it's not the same result. It will (probably) be two stops underexposed. The shutter speed hasn't changed, so the lens isn't f/2 any more, but f/4. I'm assuming that you're sufficiently up on photography to know what's happened. As you focus closer, the effective aperture of the lens changes. The value marked on the box (and the barrel) is correct, but only within the parameters under which it is defined. You'll also find (if you care to try it) that the f/2 value won't apply if you reverse the lens, as some do when using a lens close up.

Similarly, if you reverse the lens, the 2 stop exposure increase needed when working at 1:1 won't apply in most cases. The amount depends on the lens design, and the discrepancy can actually be very large.From data published by Zeiss on two of its lenses, our 4 times increase could be less than that (3.78) or nearly 3 times that figure (11.06).

The focal length works in the same way. The marked value will be accurate (to within about 1% usually) when it is measured in the way that the focal length is defined. Modern lens design (with both zoom lenses and internal focusing ones) means that the focal length will only be accurate at infinity focusing.

Since I mentioned the 1% in an earlier post, I'll just emphasise that I mean 1%, so 1mm in 100mm, 10mm in 1000mm, and not 1mm. A 1mm difference in focal length can make a whopping diffference when the focal length is very small (as with a compact digital camera, where 1mm is about 15% of the focal length).

[StephenBatey]

Having reread your previous post, I'd like to offer a further observation. Photography is based on two scientific effects; the way light can be bent by lenses to produce an image, and the effect light can have on certain objects that make the recording of that image possible. These can only be fully understood and predicted if you have some grasp of the science underlying them. And, as you've found, sometimes what is entirely predictable by science can be amazingly unexpected if you don't know the principles. As a simple example, think of the effect of observing a solar eclipse for the first time if you have no prior knowledge of them.

Sometimes you need the science to make sense of the reality. In this instance, you need it to be able to predict which lens to use, assuming you have a choice and also that the exact focal length (and hence image magnification) matters. There are some rather unexpected effects of lens design to be detected in practical work if you use a lens in certain circumstances.

I personally like to know what's going on, and most arcane pieces of knowledge do have a practical bearing when the circumstances are right. I know a lot of things that will hardly ever make any difference at all in practice most of the time; but sometimes they will; and knowing them means that I can either work around them or use them to my advantage.

Switching subjects from lenses to shutters provides an easily seen illustration. If you don't know it, find Laritgue's photo of the Grand Prix of the Paris Auto Club 1912, which is easily found on Google. The effect of the spectators leaning one way and the car the other (giving what Ansel Adams described as the most effectiive impression of motion that he knew of) was anticipated by the photographer, who used the camera in a way that would produce it. It's based on focal plane shutter distortion.

Summarising, you can get what you want from your equipment without knowing these things. But if you know them, you'll realise that your equipment can actually give you far more control than you realised, and you can achieve things that you would never have thought possible without this knowledge. It expands your horizons, and pushes your boundaries outwards. It means that you can contemplate images that you might otherwise never see as a practical possibility.

[DouglasMorley]

Fascinating information Stephen. Thanks

Owning a Tamron 90mm macro lens, I had been aware of this aperture change with focus distance though I only noticed it with really close up work, not portraiture, which I will re-examine

So maybe I can put my "problem" with the 18-70mm lens down to lens design.

[StephenBatey]

In part, yes. But the constraints on lens design that are principally responsible are the requirements for autofocus.

[StephenBatey]

Originally Posted by DouglasMorley

As to the technical and mathematical reasoning behind my original query, you are, by and large quite right in that few photographers/picture takers care as long as they get what they want from their equipment

Having reread the thread, I've made a correction to an earlier post. I also can't resist providing a comment on the sentence above, in the words of Leonardo da Vinci:

Those who are in love with practice without knowledge are like the sailor who gets into a ship without rudder or compass and never can be certain whither he is going. Practice must always be founded on sound theory.