Well, this is the part that can be done without touching or using either lens. Just the basic listing of specs to see how they compare. Note that I chose not to call it Sigma 50 vs. Tamron 90, as they are not intended to compete with each other - Sigma makes other macro lenses closer to the 90mm focal length. I just want to own one macro lens, so this comparison is intended to help me (and maybe you - no guarantees though!) to pick the most suitable of the two.
For several months now I have owned the Tamron SP 90mm f/2.8 macro. It is model 172E and not the most recent Di model 272E, but the only difference is in the coatings - Tamron itself says so. Tamron has made 90mm macro lenses for quite some time, and has been very good at it - so making major optical changes isn't likely!
I have enjoyed taking much closer close-up images with this lens, and it has delivered great results. Let's be clear about that! However, I decided that 90mm didn't quite fit my 'needs'. (BTW never ask a photographer what their 'needs' are, the answer won't really make sense to anyone else!) When I want to use AF zooms I have that covered well enough, and in primes I have a 28mm and either a 40 or 50 - several of these in fact, none of which quite meets my 'needs'. I also have a great 70-150 f/4 zoom with prime-like image quality, and that lens usurps the Tamron fairly often. Given these two concerns, I found a Sigma 50/2.8 macro that could prove to be a great fit. I bought it after two people contacted me about the 90 when I put it up for sale, so figuring it was as good as gone I made the purchase. My loss is the web's gain, as I shall have both for at least a few days to compare them!
So on to specs: check the table below for highlights. Both are f/2.8-32, both have aperture rings with the auto 'A' setting in Pentax mount, and both take 55mm filters. Both also have focus-limiting switches to speed AF for general-purpose use. Add to this the various reviews online, where for each "my lens has a lot of chromatic aberration" at least three others have very little - that goes for both lenses, as does the comment that "there are no bad macro primes".
|Model||Tamron SP 90 #172E||Sigma EX DG 50|
|Angle of View||27°||46.8º|
|Min Focus in.(m)||11.4(0.29)||7.4(0.19)|
|Max Mag. Ratio||1:1||1:1|
|Diameter x Length||2.9x3.8"||2.8x2.5"|
To no one's surprise the 50mm is smaller and lighter. It also has no manual-focus switch, that's done on the camera body. The Tamron requires the body switch plus a second push-pull cam to achieve manual focus; Canon/Nikon users need only use the cam but Sony/Pentax folks are forced to take two steps. These come across as slight wins for the Sigma 50, but nothing major. My copy is the EX DG model, which means that unlike my Tamron it does have 'for-digital' coatings; we shall see if that has meaning in my shots! They have a different number of aperture blades, which could make a difference - again consulting the web, many people swear that blades affect bokeh far less than optics so let's not get excited over this quite yet.
Many debates go on about which is the better 'portrait' lens. For those who stick to convention the 90 wins here - but on aps-c cameras it takes 135mm-scale images, which is a bit above the traditional length. The 50 is more like 75mm which is on the low end of tradition. But let's be honest: my 70-150 and 55-300 zooms are fine portrait lenses, and at f/4 in the traditional range I'll probably have the entire face in focus, not just eyes. So I have enough portrait options not to worry, and both would do well enough - so to me this debate has no clear winner and is not a factor.
One other point does have merit. To achieve 1:1 macro imaging the 90mm uses a minimum focus that is further from the subject. The 50mm crowds the subject to reach 1:1, which can cause lighting problems and will spook away many creatures with wings or fast legs. Caterpillars may not care, and flowers definitely do not - but in some cases this will matter. People affected by this are probably seeking 150-180mm macros anyway, but it's worth a paragraph and favors the Tamron 90.
So how shall I test two such disparate lenses that do the same things? It's fair to shoot a few general images from the same spot, to check both fields on their own merit as well as to see the difference generally. After that, a torture test or two for chromatic issues, then in for close shots of the same items to the same scale at a couple of apertures. This always proves to be a challenge to me, as at some point I will fail to keep the playing field level for "rigorous" tests. ISO or white balance will slip to Auto, and when I learn of it I'll be too pooped to replay the games. Oh well, I'll do my usual best & complain about it later (maybe by part 3).
Speaking for myself (and it is my blog!), my use of macro will not be a full-time passion. It will be far more common to shoot plants than bugs, and I like one less step to go from AF to MF. And as noted before, I have a 'need' for 50mm in my kit more than 90mm, whether to fit in with the 55-300 or 70-150. So it sounds like I have biased feelings here - which leads to another problem, 'new-lens syndrome'. The most common reason for buying a new lens is because something isn't quite right about the other ones. (That's not always true, I admit/confess!) In any case the newest lens always gets strong press and the benefit of the doubt... for a while at least. Despite this I will resist the temptation to declare a winner in part zero!
One more item about me is worth mentioning: I am not a big-lens fan. I have found that anything over 500 grams gets left home too often to bother with. The Tamron is only 410 grams, more or less; the Rikenon 70-150 is 40g more and I'm happy to carry that lens. The macro is quite a bit thicker though, so on some level it feels larger than it is - and that could be a subconscious factor against it for me. I'll try not to let it be so.
Finally - never ever forget that I am comparing one copy of the Tamron to one copy of the Sigma! One or the other may front focus a smidge, or have elements not precisely centered. They may both be absolutely perfect; if so I'm one lucky guy!
So - focus speed and noise, sharpness, bokeh and the like await part one of this series, when the Sigma 50mm is in my hand and on my K-5. For now I can say that the Tamron is reasonably quick to focus, a bit noisy getting there, seldom hunts, and I find it's a pain to take 2 steps to go from AF to MF. Image quality is excellent, with truly minimal chromatic issues in my cruel tests designed to reveal such issues.
And that's as much as I will say for part zero!