“Il mio obiettivo è morbido…” ed altri miti – II

I test per valutare front e back focus sul proprio corredo

Test di valutazione front back focusIntanto ricordiamo che i test sono sempre specifici per un determinato insieme corpo macchina/ obiettivo e non replicabili su accoppiamenti differenti.

Le ottiche a focale variabile potrebbero presentare il problema in modo non uniforme sulle diverse lunghezze focali, consigliamo in questo caso di concentrare le prove su un unica lunghezza focale, magari quella che utilizzate più spesso, o al limite la più lunga, maggiormente critica secondo quanto già visto nell’ articolo precedente.

Le modalità esistenti per testare la presenza di fenomeni di front-back focusing sul complesso macchina/obiettivo sono abbastanza simili fra loro, si tratta nella maggior parte dei casi di fotografare da distanza opportuna una mira ottica adeguata e verificare poi visivamente se l’immagine ottenuta rende correttamente nitidi i riferimenti centrali o se invece la nitidezza si sposta avanti o indietro rispetto alla linea di fuoco.

Vari tipi di mire sono disponibili gratuitamente in rete, da downloadare e stampare su carta, o possono essere acquistate già pronte per chi ha meno dimestichezza con il fai-da-te. Un’interessante modalità di test non prevede l’uso di mire ottiche ma la semplice fotografia dell’immagine sul monitor del PC sfruttando le peculiarità dell’Effetto Moirè.

Abbiamo raggruppato le migliori e più affidabili risorse del Web che vi invitiamo a leggere prima di procedere alle verifiche pratiche, possibilmente anche quelle in lingua inglese, sempre ricche di schemi e disegni. Come al solito nella sezione Commenti in fondo all’articolo potremo approfondire insieme eventuali difficoltà di interpretazione. Cliccare sulle immagini sottostanti per andare all’articolo corrispondente.

Iniziamo la nostra carrellata con un articolo in italiano di Daniele Pace, scritto in maniera semplice ed efficace e con la possibilità di scaricare la mira da fotografare oltre che le istruzioni per utilizzarla al meglio.

verificare-front-back-focus-di-un-obiettivo


Un ottimo articolo in inglese di Jeffrey Friedl e la sua ottima ‘Autofocus Test Chart’, con una procedura di test molto completa ed approfondita e ricco di fotografie esplicative.

autofocus_test_chart


Keith Cooper di Northlight Images propone un test molto particolare ed apprezzato, anche se di comprensione meno immediata, che non prevede la stampa di mire su carta ma la semplice fotografia di un immagine a monitor in maniera opportuna e sfruttando le particolarità dell’ Effetto Moirè. Nello stesso articolo i suggerimenti di Canon sull’argomento front-back focus ed una modalità di test alternativa che sfrutta il sistema LiveView.

moire_focus_test

Per chi invece preferisce utilizzare uno strumento già pronto per i test evitando di dover stampare la mira, incollarla su un supporto rigido ed inclinarla opportunamente, una serie di prodotti già pronti per l’uso:

Dall’italiana Fotools, distribuzione APROMA, l’AF-TOOL

pensato sia per il professionista sia per l’amatore evoluto, permettendo di risolvere velocemente i problemi di back-focusing e di front focusing, rendendo in questo modo efficace l’uso dei controlli di microregolazione dell’autofocus di cui sempre più fotocamere digitali sono dotate

aftool_fotools


Il noto LensAlign, oggi alla versione MkII, della Michael Tapes Design, uno dei tool più apprezzati dai fotografi di tutto il mondo.

lensalign_backfocus_test


SpyderLensCal™ di Datacolor, semplice ed efficace e dotato di bolla per la perfetta messa in piano dello strumento.

spyderlens_datacolor_test

Ed ora che abbiamo fatto i nostri bravi test e controllato tutta l’attrezzatura cosa succede?

Bene, se siamo stati fortunati e le variazioni riscontrate sono nulle o minime torniamo ad utilizzare la nostra fotocamera felicemente facendo magari più attenzione ad alcuni comportamenti che possono compromettere la nitidezza anche vistosamente, parliamo del micromosso e dell’uso di tempi di posa sufficientemente rapidi rispetto alla lunghezza focale utilizzata, dell’accuratezza della messa a fuoco, del corretto sfruttamento della PdC.

Se invece rileviamo scostamenti importanti, e dopo aver ripetuto più volte i test per sicurezza, ed in presenza di front o back focus possiamo procedere alla taratura fine dell’ottica se possediamo una reflex di gamma alta e dotata di questa possibilità:

Nikon – D300 D300s D700 D7000 D3 D3s D3x

Canon – Eos 50D Eos 7D Eos 5D MkII Eos 1D MkIII Eos 1D MkIV Eos 1Ds MKIII

Pentax – K7 K5

Olympus – E-30 E-620 E-5

Sony – Alpha 850 Alpha 900

Af_fine_adjustment
Il pannello di taratura fine della Pentax K-5

Ricordiamo che l’operazione è delicata e va fatta con cognizione di causa e dopo attento studio del manuale e delle procedure specifiche per ogni fotocamera.

Chi non possiede una delle reflex suddette non ha grandi possibilità di intervento; se il problema è grave si può provare a spedire l’ottica in laboratorio per una taratura (non dimenticando che l’obiettivo non è l’unico responsabile del fenomeno come visto nella prima parte dell’articolo) oppure provare a testarla con altri corpi macchina sperando in una migliore combinazione.

Vista la diffusione del problema se verificate che la vostra accoppiata macchina-obiettivo risulta perfetta o quasi, pensateci due volte prima di cambiare attrezzatura…!

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